Set PYTHON_INCLUDE_DIR and PYTHON_LIBRARY during installation

[thinking-tower]

cc @unexploredtest, @axsaucedo, Superchunk. This should fix #221.

Description

Finding Python for CMake is a known issue. This PR from PyTorch, while not directly related to fixing Python installation issues, shows a tried-and-tested way to help CMake find Python. In particular, the following lines are of interest:

# Dependencies.cmake
pycmd_no_exit(_py_inc _exitcode "import sysconfig; print(sysconfig.get_path('include'))")
pycmd_no_exit(_py_lib _exitcode "import sysconfig; print(sysconfig.get_path('stdlib'))")
# setup.py
cmake_python_library = "{}/{}".format(
        sysconfig.get_config_var("LIBDIR"),
        sysconfig.get_config_var("INSTSONAME"))
cmake_python_include_dir = sysconfig.get_path("include")

I opted for the first option, as the second option gave an incorrect directory on my machine (but the installation still worked).

Notes

It's worth noting that FindPython3 (for CMake 3.12+) does a similar thing. See here