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

@@ -86,7 +86,7 @@ class build_ext(_build_ext.build_ext):
          "generate debug information for cygdb"),
         ('cython-compile-time-env', None,
             "cython compile time environment"),
-            
+
         # For backwards compatibility.
         ('pyrex-cplus', None,
          "generate C++ source files"),
@@ -109,7 +109,7 @@ class build_ext(_build_ext.build_ext):
     boolean_options.extend([
         'cython-cplus', 'cython-create-listing', 'cython-line-directives',
         'cython-c-in-temp', 'cython-gdb',
-        
+
         # For backwards compatibility.
         'pyrex-cplus', 'pyrex-create-listing', 'pyrex-line-directives',
         'pyrex-c-in-temp', 'pyrex-gdb',
@@ -127,7 +127,7 @@ class build_ext(_build_ext.build_ext):
         self.cython_gdb = False
         self.no_c_in_traceback = 0
         self.cython_compile_time_env = None
-    
+
     def __getattr__(self, name):
         if name[:6] == 'pyrex_':
             return getattr(self, 'cython_' + name[6:])
@@ -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)