bpo-41916: allow cross-compiled python to have -pthread set for CXX

[virtuald]

When cross-compiling, the compile/run test for -pthread always fails so -pthread will never be automatically set without an override from the cache. ac_cv_pthread can already be overridden, so do the same thing for ac_cv_cxx_thread.

I've validated that my cross-compiled python has the variables set now when configured via:

   ./configure --host=$TARGET_HOST --build=$BUILD_HOST --prefix=$PREFIX \
        --disable-ipv6 --enable-unicode=ucs4 \
        ac_cv_file__dev_ptmx=no ac_cv_file__dev_ptc=no \
        ac_cv_have_long_long_format=yes \
        ac_cv_pthread_is_default=no ac_cv_pthread=yes ac_cv_cxx_thread=yes; \

I now get as expected:

>>> import pprint, distutils.sysconfig
>>> pprint.pprint({i: distutils.sysconfig.get_config_vars(i)[0] for i in ('CC', 'CXX', 'LDSHARED')})
{'CC': 'arm-frc2020-linux-gnueabi-gcc -pthread',
 'CXX': 'arm-frc2020-linux-gnueabi-c++ -pthread',
 'LDSHARED': 'arm-frc2020-linux-gnueabi-gcc -pthread -shared'}

I haven't yet checked to see if it all works quite yet, will do later today.

https://bugs.python.org/issue41916

[virtuald]

Some testing indicates that this worked without issues. Given that the tests all seem to pass on CI, I think this is good to go.

[virtuald]

I have confirmed that this does fix the issues I was running into, and my cross compiled packages seem to be fine.

[FFY00]

This fix looks correct. @jaraco could you have a look?

[virtuald]

I've been using it to build python 3.9.x and more recently python 3.10.x without any issues.

[FFY00]

Perhaps, just add a news item to make it quicker to merge. I think the code is good, it is just missing a news entry.

[kumaraditya303]

You can use https://blurb-it.herokuapp.com/ to add a news fragment.

[jaraco]

I'm afraid I don't have a lot of experience with this aspect of CPython. Perhaps @ambv could review or suggest a reviewer?

[virtuald]

FWIW, I've been using the patch to cross-compile python for use in CI since I've made the patch.

[virtuald]

Rebased on main; integrated changes that were made in #29485

[virtuald]

@tiran seems to make a lot of changes to configure.ac, perhaps they might be a better reviewer for this?

[FFY00]

@tiran, do have time to have a quick look at this PR? Thanks!

[ericsnowcurrently]

IIRC, @gpshead has some interest in cross-compilation and @erlend-aasland has experience with the configure script.