meson: deprecate harder

[JakeHillion]

Now we've had the warning about Meson for a while and everyone we know about has migrated to pure Cargo, increase the level of the deprecation. The next step is to hard error the build unless a flag is specified to override this. This new flag to hide this error is documented and will force anyone who was taking updates automatically to notice the change.

Test plan:

• CI
• CI with the first version of this commit (no flag). It failed with the expected error.

[sirlucjan]

- without new flag:

==> Starting prepare()...
58e6680c (origin/meson-haters-club) meson: deprecate harder
==> Starting pkgver()...
==> Updated version: scx-scheds-git 1.0.17.r144.g737b9b13-1
==> Starting build()...
+ exec meson setup --prefix /usr --libexecdir lib --sbindir bin --buildtype plain --auto-features enabled --wrap-mode nodownload -D b_pie=true -D python.bytecompile=1 . build -D openrc=disabled -D libbpf_a=disabled -D bpftool=disabled -D b_lto=true -D b_lto_mode=thin -D cargo_home=/home/lucjan/Pobrane/scx-scheds-git2/src/scx
The Meson build system
Version: 1.9.1
Source dir: /home/lucjan/Pobrane/scx-scheds-git2/src/scx
Build dir: /home/lucjan/Pobrane/scx-scheds-git2/src/scx/build
Build type: native build
Project name: sched_ext schedulers
Project version: 1.0.17
C compiler for the host machine: cc (gcc 15.2.1 "cc (GCC) 15.2.1 20250813")
C linker for the host machine: cc ld.bfd 2.45.0
Host machine cpu family: x86_64
Host machine cpu: x86_64

meson.build:9:2: ERROR: Problem encountered: Meson builds are deprecated. Please switch to `cargo build` for Rust schedulers and `make` for C schedulers. Meson will be removed in an upcoming release. Please report any issues packaging for distributions at https://github.com/sched-ext/scx/discussions/2731. If you need Meson to unblock yourself temporarily, add `-Dforce_meson=true` when configuring.

A full log can be found at /home/lucjan/Pobrane/scx-scheds-git2/src/scx/build/meson-logs/meson-log.txt
==> ERROR: A failure occurred in build().
    Aborting...

- with new flag

+ exec meson setup --prefix /usr --libexecdir lib --sbindir bin --buildtype plain --auto-features enabled --wrap-mode nodownload -D b_pie=true -D python.bytecompile=1 . build -D openrc=disabled -D libbpf_a=disabled -D bpftool=disabled -D b_lto=true -D b_lto_mode=thin -D force_meson=true -D cargo_home=/home/lucjan/Pobrane/scx-scheds-git/src/scx
The Meson build system
Version: 1.9.1
Source dir: /home/lucjan/Pobrane/scx-scheds-git/src/scx
Build dir: /home/lucjan/Pobrane/scx-scheds-git/src/scx/build
Build type: native build
Project name: sched_ext schedulers
Project version: 1.0.17
C compiler for the host machine: cc (gcc 15.2.1 "cc (GCC) 15.2.1 20250813")
C linker for the host machine: cc ld.bfd 2.45.0
Host machine cpu family: x86_64
Host machine cpu: x86_64
Program clang found: YES (/usr/bin/clang)
Program /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/veristat found: YES (/home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/veristat)
Program /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/veristat_diff found: YES (/home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/veristat_diff)
Program /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/run_stress_tests found: YES (/home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/run_stress_tests)
Program /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/get_clang_ver found: YES (/home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/get_clang_ver)
Program /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/get_bpftool_ver found: YES (/home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/get_bpftool_ver)
Program /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/bpftool_build_skel found: YES (/home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/bpftool_build_skel)
Program /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/bpftool_build_skel_lib found: YES (/home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/bpftool_build_skel_lib)
Program /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/get_sys_incls found: YES (/home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/get_sys_incls)
Program /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/test_sched found: YES (/home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/test_sched)
Program /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/fetch_libbpf found: YES (/bin/bash /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/fetch_libbpf)
Program /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/build_libbpf found: YES (/home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/build_libbpf)
Program /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/fetch_bpftool found: YES (/bin/bash /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/fetch_bpftool)
Program /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/build_bpftool found: YES (/bin/bash /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/build_bpftool)
Program /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/compile_scx_lib found: YES (/home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/compile_scx_lib)
Program jq found: YES (/usr/bin/jq)
Program make found: YES (/usr/bin/make)
Program nproc found: YES (/usr/bin/nproc)
Program protoc found: YES (/usr/bin/protoc)
Program llvm-strip found: YES (/usr/bin/llvm-strip)
Found pkg-config: YES (/usr/bin/pkg-config) 2.5.1
Run-time dependency libseccomp found: YES 2.5.6
Run-time dependency libbpf found: YES 1.5.1
Program bpftool found: YES (/usr/bin/bpftool)
Message: cpu=x86_64 bpf_base_cflags=['-g', '-O2', '-Wall', '-Wno-compare-distinct-pointer-types', '-D__TARGET_ARCH_x86', '-mcpu=v3', '-mlittle-endian', '-idirafter /usr/lib/clang/21/include', '-idirafter /usr/local/include', '-idirafter /usr/include']
Program cargo found: YES (/usr/bin/cargo)
Program /home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/cargo_fetch found: YES (/home/lucjan/Pobrane/scx-scheds-git/src/scx/meson-scripts/cargo_fetch)
Run-time dependency threads found: YES
Dependency threads found: YES unknown (cached)
Run-time dependency systemd found: YES 258
Dependency openrc skipped: feature openrc disabled
Build targets in project: 76

sched_ext schedulers 1.0.17

  User defined options
    auto_features     : enabled
    b_lto             : true
    b_lto_mode        : thin
    b_pie             : true
    bpftool           : disabled
    buildtype         : plain
    cargo_home        : /home/lucjan/Pobrane/scx-scheds-git/src/scx
    force_meson       : true
    libbpf_a          : disabled
    libexecdir        : lib
    openrc            : disabled
    prefix            : /usr
    python.bytecompile: 1
    sbindir           : bin
    wrap_mode         : nodownload

Found ninja-1.13.1 at /usr/bin/ninja

Generating targets:   0%|          | 0/76 eta ?
                                               

Writing build.ninja:   0%|          | 0/177 eta ?
                                                 
ninja: Entering directory `/home/lucjan/Pobrane/scx-scheds-git/src/scx/build'
[1/47] Generating libbpf with a custom command

[2/47] Generating bpftool_target with a custom command

[3/47] Generating copy dsq_lat with a custom command
[4/47] Generating copy runq_lat with a custom command
[5/47] Generating copy stress wrapper with a custom command
[6/47] Compiling C object cc_cflags_probe.p/meson-scripts_cc_cflags_probe.c.o
[7/47] Linking target cc_cflags_probe
[8/47] Generating 'scheds/c/scx_prev.p/scx_prev.bpf.o'
[9/47] Generating 'scheds/c/scx_pair.p/scx_pair.bpf.o'
[10/47] Generating from 'scx_prev.bpf.o'
[11/47] Generating 'scheds/c/scx_nest.p/scx_nest.bpf.o'
[12/47] Generating 'scheds/c/scx_qmap.p/scx_qmap.bpf.o'
[13/47] Generating from 'scx_pair.bpf.o'
[14/47] Generating 'scheds/c/scx_simple.p/scx_simple.bpf.o'
[15/47] Generating 'scheds/c/scx_flatcg.p/scx_flatcg.bpf.o'
[16/47] Generating lib/sdt_task with a custom command
[17/47] Generating from 'scx_nest.bpf.o'
[18/47] Generating 'scheds/c/scx_central.p/scx_central.bpf.o'
[19/47] Generating 'scheds/c/scx_userland.p/scx_userland.bpf.o'
[20/47] Generating from 'scx_simple.bpf.o'
[21/47] Generating 'scheds/c/scx_sdt.p/scx_sdt.bpf.o'
[22/47] Generating from 'scx_qmap.bpf.o'
[23/47] Compiling C object scheds/c/scx_pair.p/scx_pair.c.o
[24/47] Compiling C object scheds/c/scx_prev.p/scx_prev.c.o
[25/47] Generating from 'scx_flatcg.bpf.o'
[26/47] Generating from 'scx_central.bpf.o'
[27/47] Generating from 'scx_userland.bpf.o'
[28/47] Compiling C object scheds/c/scx_simple.p/scx_simple.c.o
[29/47] Generating lib/sdt_alloc with a custom command
[30/47] Compiling C object scheds/c/scx_qmap.p/scx_qmap.c.o
[31/47] Compiling C object scheds/c/scx_nest.p/scx_nest.c.o
[32/47] Generating lib/lib with a custom command
[33/47] Compiling C object scheds/c/scx_central.p/scx_central.c.o
[34/47] Compiling C object scheds/c/scx_userland.p/scx_userland.c.o
[35/47] Compiling C object scheds/c/scx_flatcg.p/scx_flatcg.c.o
[36/47] Linking target scheds/c/scx_prev
[37/47] Linking target scheds/c/scx_simple
[38/47] Linking target scheds/c/scx_pair
[39/47] Linking target scheds/c/scx_qmap
[40/47] Linking target scheds/c/scx_central
[41/47] Linking target scheds/c/scx_nest
[42/47] Linking target scheds/c/scx_flatcg
[43/47] Generating from 'scx_sdt.bpf.o'
[44/47] Linking target scheds/c/scx_userland
[45/47] Compiling C object scheds/c/scx_sdt.p/scx_sdt.c.o
[46/47] Linking target scheds/c/scx_sdt
[47/47] Generating rust_all with a custom command (wrapped by meson to set env)
INFO: autodetecting backend as ninja
INFO: calculating backend command to run: /usr/bin/ninja -C /home/lucjan/Pobrane/scx-scheds-git/src/scx/build