Ensure that extension's include_dirs is a list
Ensure that extension's include_dirs is a list

This change allows to pass generator (e.g. itertools.chain) as
include_dirs if for some reason evaluation of include directories needs
to be delayed e.g. due to simultaneous dependency on cython and numpy.

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