Log Message:
modular-xorg-xquartz, libAppleWM: Use predefined OSX_SDK_PATH

The latest macOS Big Sur 11.2.1 and Xcode 12.4 as of 2021-02-24 do not
provide 11.2 SDK. They provide 11.1 SDK instead. mk/platform/Darwin.mk has
workaround for this situation. So use the workaround directly.

OSX_SDK_PATH may be internal, however it is accessible over 5 years.
It may not be a big problem hopefully.