support building of C extensions - rewrite

[gaborbernat]

With the rewrite branch, let's ensure #1023 is supported. The hunch is we might need the header files but we need to validate.

[gaborbernat]

As the test library, we probably can use https://github.com/sumerc/yappi 👍

[gaborbernat]

Where I've got is that pip will inject the virtual environment include folder onto the include path, however, can there be any content there? Standard library headers are already added as part of the standard library path; so the question feels to me goes towards do any package install headers that would as such not be available globally, but only within a virtual environment?

clang -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -I/private/var/folders/kt/btg285ds5kx4l1k398lb7df80000gr/T/pytest-of-bgabor8/pytest-228/test_can_build_c_extensions_cp0/include -I/Users/bgabor8/.pyenv/versions/3.8.1/include/python3.8 -c yappi/_yappi.c -o build/temp.macosx-10.14-x86_64-3.8/yappi/_yappi.o

[gaborbernat]

Now works.