Warn about multiple Cython sources in a single extension.
Warn about multiple Cython sources in a single extension.

scoder/cython,scoder/cython,fabianrost84/cython,fabianrost84/cython,scoder/cython,scoder/cython,cython/cython,cython/cython,cython/cython,fabianrost84/cython,fabianrost84/cython,cython/cython,da-woods/cython,da-woods/cython,ChristopherHogan/cython,ChristopherHogan/cython,da-woods/cython,ChristopherHogan/cython,da-woods/cython,fabianrost84/cython