gh-159603: Use pthread_attr_get_np() on FreeBSD

[vstinner]

Use pthread_attr_get_np() on FreeBSD to get the stack size (and guard size) of the current thread.

Check if pthread_attr_get_np() function and <pthread_np.h> header are available in configure.

• Issue: Implement stack size detection on FreeBSD #159603

[vstinner]

Sadly, I added the new HAVE_PTHREAD_NP_H macro to Tools/build/check_capi_macros_ignored.txt. I don't know how to check if a header is available in configure and define a macro name different than HAVE_<header>_H.

I asked on #157696 (comment) if there is a way to define a _Py_HAVE_xxx_H macro instead.

[vstinner]

Without this change, test_lazy_import.test_deeply_dotted_name() does crash with SIGILL on FreeBSD, see: #159535.

With this change, the test pass as expected (I tested FreeBSD 16):

$ ./python -m test test_lazy_import -v -m test_deeply_dotted_name
== CPython 3.16.0a1+dev (heads/pr/159604:c031a553097, Oct 17 2026, 02:48:37) [Clang 21.1.8 (https://github.com/llvm/llvm-project.git llvmorg-21.1.8-0-g2078da
== FreeBSD-16.0-CURRENT-amd64-64bit-ELF little-endian
== Python build: debug
(...)
test_deeply_dotted_name (test.test_lazy_import.LazyImportTypeTests.test_deeply_dotted_name)
A long dotted name must not overflow the C stack. ... ok
(...)
Result: SUCCESS

[vstinner]

!buildbot FreeBSD16

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @vstinner for commit c031a55 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F159604%2Fmerge

The command will test the builders whose names match following regular expression: FreeBSD16

The builders matched are:

• AMD64 FreeBSD16 Non-Debug PR
• AMD64 FreeBSD16 PR

[vstinner]

Tail calling interpreter / x86_64-apple-darwin/clang (pull_request): Cancelled after 60m

I created #159612 for this unrelated failure.