setup: look in multiarch paths in GNU platforms

Somehow I merged pino's commit into rel_2.3 branch (which I since removed because it was confusing me). Not sure what happened, but this is his code that got lost.

setup.py

@@ -267,6 +267,9 @@ def build_extensions(self):
             # work ;-)
             self.add_multiarch_paths()
 
+        elif sys.platform.startswith("gnu"):
+            self.add_multiarch_paths()
+
         elif sys.platform.startswith("netbsd"):
                     _add_directory(library_dirs, "/usr/pkg/lib")
                     _add_directory(include_dirs, "/usr/pkg/include")