quartz: fix build on 10.6 and below

It is not possible to successfully build GTK+ on OS X 10.6 and below
since NSFullScreenWindowMask is only available starting with 10.7. Add
ifdef guards around setStyleMask: in order to allow it to build on
earlier OS X releases.

https://bugzilla.gnome.org/show_bug.cgi?id=737561

gdk/quartz/GdkQuartzNSWindow.h

@@ -42,7 +42,10 @@
 -(BOOL)trackManualResize;
 -(void)showAndMakeKey:(BOOL)makeKey;
 -(void)hide;
+
+#ifdef AVAILABLE_MAC_OS_X_VERSION_10_7_AND_LATER
 -(void)setStyleMask:(NSUInteger)styleMask;
+#endif
 
 @end