add post-messages to conf-libpcre for debian failure

[raphael-proust]

Because of the failure observed in #28754

  * Missing dependency:
    - passage >= 0.1.8 -> devkit >= 1.20240429 -> pcre -> conf-libpcre
    depends on the unavailable system package 'libpcre3-dev'. Use `--no-depexts' to attempt installation anyway, or it is possible that a depext package name in the opam file is incorrect.

[jmid]

Good idea!

Unrelatedly, there is something fishy with conf-libpcre on MSys2 MinGW:

  #=== ERROR while compiling conf-libpcre.2 =====================================#
  # context     2.4.1 | win32/x86_64 | ocaml.5.3.0 | file://D:/a/opam-repository/opam-repository
  # path        D:\opamroot\default\.opam-switch\build\conf-libpcre.2
  # command     C:\msys64\mingw64\bin\pkg-config.exe --print-errors --exists libpcre
  # exit-code   1
  # env-file    D:\opamroot\log\conf-libpcre-2588-a9ff34.env
  # output-file D:\opamroot\log\conf-libpcre-2588-a9ff34.out
  ### output ###
  # Package libpcre was not found in the pkg-config search path.
  # Perhaps you should add the directory containing `libpcre.pc'
  # to the PKG_CONFIG_PATH environment variable
  # Package 'libpcre' not found

However the intention is to run
pkgconf --personality=x86_64-w64-mingw32 --print-errors --exists libpcre there, not
pkg-config.exe --print-errors --exists libpcre 🤔