Need setup.py to pick up -isystem flags from CPPFLAGS

[jherland]

BPO	37763
Nosy	@moreati, @jherland
PRs	bpo-37763: Teach setup.py to pick up "-isystem <dir>" from $CPPFLAGS #15136

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-08-05.15:10:05.216>
labels = ['type-feature', '3.8', '3.9', 'build']
title = 'Need setup.py to pick up -isystem flags from CPPFLAGS'
updated_at = <Date 2019-08-05.15:15:28.475>
user = 'https://github.com/jherland'

bugs.python.org fields:

activity = <Date 2019-08-05.15:15:28.475>
actor = 'jherland'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Cross-Build']
creation = <Date 2019-08-05.15:10:05.216>
creator = 'jherland'
dependencies = []
files = []
hgrepos = []
issue_num = 37763
keywords = ['patch']
message_count = 1.0
messages = ['349054']
nosy_count = 2.0
nosy_names = ['Alex.Willmer', 'jherland']
pr_nums = ['15136']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue37763'
versions = ['Python 3.8', 'Python 3.9']

[jherland]

First time contributor here, still learning the ropes.

We're cross-compiling python in an environment where we set up CPPFLAGS, LDFLAGS, etc. to point directly to the locations where we have built Python's dependencies. For example, we will typically build Python in an environment that includes:

CPPFLAGS=\
-isystem /path/to/ncurses/build/include \
-isystem /path/to/libffi/build/include \
-isystem /path/to/zlib/build/include \
-isystem /path/to/openssl/build/include \
-isystem /path/to/readline/build/include

LDFLAGS=\
-L/path/to/ncurses/build/lib \
-L/path/to/libffi/build/lib \
-L/path/to/zlib/build/lib \
-L/path/to/openssl/build/lib \
-L/path/to/ciscossl-fom/build/lib \
-L/path/to/readline/build/lib

setup.py already picks up our -L options from LDFLAGS and propagates them into the build commands, but the -isystem options from CPPFLAGS are currently ignored.

I will post a PR that teaches setup.py to handle -isystem options in CPPFLAGS the same way it currently handles -I options.

[serhiy-storchaka]

setup.py no longer used to build CPython since 3.12 (see #94474).