Enable GCC to find free-threaded python DLL library

[andfoy]

This fixes a case not covered in PR #13338, which causes Meson not to locate the Python free-threaded DLL libraries, whose suffix ends in t when using GCC.

cc @rgommers

[rgommers]

Thanks for this fix @andfoy! This is clearly correct, not sure how I missed that in the previous PR. The one CI failure is clearly unrelated, so this should be good to merge.

I re-checked this whole function, and there is also:

            if self.static:
                libpath = Path('libs') / f'libpython{vernum}.a'

That doesn't seem to be used though. The official CPython installer only contains libpython313.lib and libpython313t.lib, no .a. The if-else could be added there too, or left out until there's a use case, no strong opinions either way.

I'll add the need for CI to gh-13263 (and work on that issue); we already discussed whether to test with MinGW in addition to MSVC there.

This does fix the SciPy wheel build on Windows, which uses MinGW, so it'd be great to get this in and backported for Meson 1.6.1.