GH-128469: warn when libpython was loaded from outside the build directory

[FFY00]

• Issue: Running Python from the build directory can lead to wrong libpython being loaded #128469

[eli-schwartz]

Does it make sense to preemptively error out here and force the user to set up LD_LIBRARY_PATH or a similar mechanism?

The other approach (rpath) was rejected for requiring multiple link jobs but would have enforced the correct library. Making it an error to use the wrong library feels a bit closer in spirit to that. And I'm not sure there's a valid use case for people building a worktree python but loading the wrong library, except when messing up -- and the consequences could be strange forms of crashing.

[FFY00]

I feel like a warning is enough for now. I think erroring out may possibly be a good next step, but I want to see how this impacts the development workflow for other folks before turning it into a hard error.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot ARM64 macOS 3.x has failed when building commit 31c82c2.

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/#/builders/725/builds/9632) and take a look at the build logs.
4. Check if the failure is related to this commit (31c82c2) 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/#/builders/725/builds/9632

Failed tests:

• test.test_multiprocessing_spawn.test_processes

Failed subtests:

• test_repr_rlock - test.test_multiprocessing_spawn.test_processes.WithProcessesTestLock.test_repr_rlock

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

==

Click to see traceback logs

Traceback (most recent call last):
  File "/Users/buildbot/buildarea/3.x.pablogsal-macos-m1.macos-with-brew/build/Lib/test/_test_multiprocessing.py", line 1530, in test_repr_rlock
    self.assertEqual('<RLock(SomeOtherThread, nonzero)>', repr(lock))
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: '<RLock(SomeOtherThread, nonzero)>' != '<RLock(None, 0)>'
- <RLock(SomeOtherThread, nonzero)>
+ <RLock(None, 0)>