bpo-31904: add library search path by wr-cc in add_cross_compiling_paths()

[pxinwr]

VxWorks has a compiler driver utility called wr-cc which wraps gcc or clang compiler. The call of sysconfig.get_config_var('CC') on building machine returns "wr-cc" for VxWorks cross compiling. "wr-cc --print-search-dirs" will add VSB specific path for library searching. Building extension modules will need these paths. So adding them in add_cross_compiling_paths().

https://bugs.python.org/issue31904

[vstinner]

LGTM, thanks for the updates.

I'm not fully comfortable with this PR which is maybe a little bit too specific to "clang --print-search-dirs" on Windows using MSYS. It's only used on VxWorks and only for cross-compilation. It's a little bit annoying to have to accumulate such code in setup.py to support different toolchains, but I don't see a better option for now.

distutils is not great for cross-compiling, and right now setup.py is implemented with distutils build_ext and Extension classes. I doubt that setuptools will have a better support for cross-compilation soon.

In the meanwhile, setup.py is the right place for such code.

[vstinner]

I rewrote the commit title to better summarize what it does. Thanks @pxinwr, I merged your PR.