Clang: Add -isystem flag support everywhere

In GNU.cmake, -isystem is not used if APPLE is set. However, Clang has
pretty much always supported -isystem, so we should always use it.

In the future, GNU.cmake should do a version check to see if -isystem is
supported.
Merge topic 'dev/add-isystem-for-clang' into next

4d668f3 Clang: Add -isystem flag support everywhere