dev-python/matplotlib: fix Darwin build

* -Werror=unguarded-availability doesn't exist on GCC
* We can't link against Cocoa right now with GCC anyway
  (missing nullable support)

URL: https://github.com/matplotlib/matplotlib/blob/7d03d8b34a218c1a9572035d4d9de170fc1163bc/setupext.py
Package-Manager: Portage-3.0.9, Repoman-3.0.2
Signed-off-by: Sam James <sam@gentoo.org>

dev-python/matplotlib/matplotlib-3.3.3.ebuild

@@ -201,6 +201,7 @@ python_configure() {
 		agg = True
 		gtk = False
 		gtkagg = False
+		macosx = False
 		pyside = False
 		pysideagg = False
 		qt4 = False