BUG: distutils: fix building mixed C/Fortran extensions

[charris]

Backport of #20881.

In SciPy we had a couple of cases where we build a Python extension with C source files but linked against static libraries built from Fortran code. Those should be using the Fortran linker, but this was broken in 1.22.0 by gh-19713 (the PR that introduced C++ in NumPy).

This fixes a few issues in the build_ext command, and documents better what is going on there.

Should close SciPy issues scipy/scipy#8325 and scipy/scipy#15414.

In the build log, this should show up:

WARN: resetting extension 'scipy.integrate._quadpack' language from 'c' to 'f77'.

And then the resetting should actually happen. So not this:

INFO: building 'scipy.integrate._quadpack' extension
INFO: compiling C sources
INFO: C compiler: /home/rgommers/anaconda3/envs/scipy-dev/bin/x86_64-conda-linux-gnu-cc -Wno-unused-result -Wsign-compare -DNDEBUG -fwrapv -O2 -Wall -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem /home/rgommers/anaconda3/envs/scipy-dev/include -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem /home/rgommers/anaconda3/envs/scipy-dev/include -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem /home/rgommers/anaconda3/envs/scipy-dev/include -DNDEBUG -D_FORTIFY_SOURCE=2 -O2 -isystem /home/rgommers/anaconda3/envs/scipy-dev/include -fPIC

INFO: compile options: '-DHAVE_CBLAS -Iscipy/_lib/src -I/home/rgommers/anaconda3/envs/scipy-dev/lib/python3.9/site-packages/numpy/core/include -Ibuild/src.linux-x86_64-3.9/numpy/distutils/include -I/home/rgommers/anaconda3/envs/scipy-dev/include/python3.9 -c'
extra options: '-msse -msse2 -msse3'
INFO: x86_64-conda-linux-gnu-cc: scipy/integrate/_quadpackmodule.c
 
INFO: /home/rgommers/anaconda3/envs/scipy-dev/bin/x86_64-conda-linux-gnu-cc -shared -Wl,-O2 -Wl,--sort-common -Wl,--as-needed -Wl,-z,relro -Wl,-z,now -Wl,--disable-new-dtags -Wl,--gc-sections -Wl,-rpath,/home/rgommers/anaconda3/envs/scipy-dev/lib -Wl,-rpath-link,/home/rgommers/anaconda3/envs/scipy-dev/lib -L/home/rgommers/anaconda3/envs/scipy-dev/lib -Wl,-O2 -Wl,--sort-common -Wl,--as-needed -Wl,-z,relro -Wl,-z,now -Wl,--disable-new-dtags -Wl,--gc-sections -Wl,-rpath,/home/rgommers/anaconda3/envs/scipy-dev/lib -Wl,-rpath-link,/home/rgommers/anaconda3/envs/scipy-dev/lib -L/home/rgommers/anaconda3/envs/scipy-dev/lib -Wl,-O2 -Wl,--sort-common -Wl,--as-needed -Wl,-z,relro -Wl,-z,now -Wl,--disable-new-dtags -Wl,--gc-sections -Wl,-rpath,/home/rgommers/anaconda3/envs/scipy-dev/lib -Wl,-rpath-link,/home/rgommers/anaconda3/envs/scipy-dev/lib -L/home/rgommers/anaconda3/envs/scipy-dev/lib -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem /home/rgommers/anaconda3/envs/scipy-dev/include -DNDEBUG -D_FORTIFY_SOURCE=2 -O2 -isystem /home/rgommers/anaconda3/envs/scipy-dev/include build/temp.linux-x86_64-3.9/scipy/integrate/_quadpackmodule.o -L/home/rgommers/anaconda3/envs/scipy-dev/lib -Lbuild/temp.linux-x86_64-3.9 -lquadpack -lmach -lopenblas -lopenblas -o scipy/integrate/_quadpack.cpython-39-x86_64-linux-gnu.so -Wl,--version-script=build/temp.linux-x86_64-3.9/link-version-scipy.integrate._quadpack.map

But this:

INFO: building 'scipy.integrate._quadpack' extension
INFO: compiling C sources
INFO: C compiler: /home/rgommers/anaconda3/envs/scipy-dev/bin/x86_64-conda-linux-gnu-cc -Wno-unused-result -Wsign-compare -DNDEBUG -fwrapv -O2 -Wall -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem /home/rgommers/anaconda3/envs/scipy-dev/include -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem /home/rgommers/anaconda3/envs/scipy-dev/include -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem /home/rgommers/anaconda3/envs/scipy-dev/include -DNDEBUG -D_FORTIFY_SOURCE=2 -O2 -isystem /home/rgommers/anaconda3/envs/scipy-dev/include -fPIC

INFO: compile options: '-DHAVE_CBLAS -Iscipy/_lib/src -I/home/rgommers/code/numpy/numpy/core/include -Ibuild/src.linux-x86_64-3.9/numpy/distutils/include -I/home/rgommers/anaconda3/envs/scipy-dev/include/python3.9 -c'
extra options: '-msse -msse2 -msse3'
INFO: x86_64-conda-linux-gnu-cc: scipy/integrate/_quadpackmodule.c

INFO: /home/rgommers/anaconda3/envs/scipy-dev/bin/x86_64-conda-linux-gnu-gfortran -Wall -g -Wall -g -shared -Wl,-O2 -Wl,--sort-common -Wl,--as-needed -Wl,-z,relro -Wl,-z,now -Wl,--disable-new-dtags -Wl,--gc-sections -Wl,-rpath,/home/rgommers/anaconda3/envs/scipy-dev/lib -Wl,-rpath-link,/home/rgommers/anaconda3/envs/scipy-dev/lib -L/home/rgommers/anaconda3/envs/scipy-dev/lib build/temp.linux-x86_64-3.9/scipy/integrate/_quadpackmodule.o -L/home/rgommers/anaconda3/envs/scipy-dev/lib -L/home/rgommers/anaconda3/envs/scipy-dev/bin/../lib/gcc/x86_64-conda-linux-gnu/9.4.0/../../../../x86_64-conda-linux-gnu/lib/../lib -L/home/rgommers/anaconda3/envs/scipy-dev/bin/../lib/gcc/x86_64-conda-linux-gnu/9.4.0/../../../../x86_64-conda-linux-gnu/lib/../lib -Lbuild/temp.linux-x86_64-3.9 -lquadpack -lmach -lopenblas -lopenblas -lgfortran -o scipy/integrate/_quadpack.cpython-39-x86_64-linux-gnu.so -Wl,--version-script=build/temp.linux-x86_64-3.9/link-version-scipy.integrate._quadpack.map