Add gamepad mapping filtering by backend

This fixes gamepad mappings being included in GLFW binaries that would
then never use them because they were for a different platform.

src/cocoa_joystick.h

@@ -33,6 +33,7 @@
 #define _GLFW_PLATFORM_LIBRARY_JOYSTICK_STATE struct { int dummyJoystick; }
 
 #define _GLFW_PLATFORM_MAPPING_NAME "Mac OS X"
+#define GLFW_BUILD_COCOA_MAPPINGS
 
 // Cocoa-specific per-joystick data
 //