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.

Cython/Distutils/build_ext.py

@@ -236,6 +236,10 @@ class build_ext(_build_ext.build_ext):
                     includes.append(i)
         except AttributeError:
             pass
+
+        # In case extension.include_dirs is a generator, evaluate it and keep
+        # result
+        extension.include_dirs = list(extension.include_dirs)
         for i in extension.include_dirs:
             if not i in includes:
                 includes.append(i)