Metal: Enable only when building for 10.11.

src/config.h

@@ -43,8 +43,8 @@
 
 #	ifndef BGFX_CONFIG_RENDERER_METAL
 #		define BGFX_CONFIG_RENDERER_METAL (0 \
-					|| BX_PLATFORM_IOS \
-					|| BX_PLATFORM_OSX \
+					||  BX_PLATFORM_IOS \
+					|| (BX_PLATFORM_OSX >= 101100) \
 					? 1 : 0)
 #	endif // BGFX_CONFIG_RENDERER_METAL