Ensure setup.py looks for zlib.h in an OS X SDK.

setup.py

@@ -1313,6 +1313,8 @@ class PyBuildExt(build_ext):
             zlib_h = zlib_inc[0] + '/zlib.h'
             version = '"0.0.0"'
             version_req = '"1.1.3"'
+            if host_platform == 'darwin' and is_macosx_sdk_path(zlib_h):
+                zlib_h = os.path.join(macosx_sdk_root(), zlib_h[1:])
             with open(zlib_h) as fp:
                 while 1:
                     line = fp.readline()