bpo-45881: Use CC from env first for cross building (GH-29752)

[tiran]

setup.py now uses CC from environment first to discover multiarch
and cross compile paths. The sysconfig variable contains the value for
the build host.

The patch also re-arranges set_compiler_executables,
configure_compiler, and init_inc_lib_dirs to make the code
easier to follow.

Signed-off-by: Christian Heimes [email protected]

https://bugs.python.org/issue45881

[miss-islington]

Thanks @tiran for the PR 🌮🎉.. I'm working now to backport this PR to: 3.9, 3.10.
🐍🍒⛏🤖

[miss-islington]

Sorry, @tiran, I could not cleanly backport this to 3.10 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker b30bf4520ae9d6e7eca09d812dd8a86c020b9202 3.10

[miss-islington]

Sorry @tiran, I had trouble checking out the 3.9 backport branch.
Please backport using cherry_picker on command line.
cherry_picker b30bf4520ae9d6e7eca09d812dd8a86c020b9202 3.9

[bedevere-bot]

GH-29754 is a backport of this pull request to the 3.9 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot s390x Fedora Clang Installed 3.10 has failed when building commit 545aebd.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/690/builds/488) and take a look at the build logs.
4. Check if the failure is related to this commit (545aebd) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/690/builds/488

Summary of the results of the build (if available):

Click to see traceback logs

fatal: unable to access 'https://github.com/python/cpython.git/': The requested URL returned error: 504

chmod: cannot access 'target/': No such file or directory

make: *** No rule to make target 'distclean'.  Stop.