generate Cython C files during build
    
    Portmidi ships an outdated py2-setup.py which we can't use and a
    py2-pyrex file which compiles under a py3-cython -2, so we do the cython
    and gcc steps by hand. (The pyrex file needs a patch to compile with
    cython 0.21 or newer, be it py2 or py3.)