BLD : fix build on windows

patch suggested by cgohlke to fix issues with building
FreeType extension

Closes matplotlib#3140

Author: tacaswell

setupext.py

@@ -918,6 +918,8 @@ def add_flags(self, ext):
             alt_exec='freetype-config')
 
     def get_extension(self):
+        if sys.platform == 'win32':
+            return None
         ext = make_extension('freetype2', [])
         self.add_flags(ext)
         return ext