Add isystem flag support if using clang

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

2a4e020 Add isystem flag support if using clang