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.

Modules/Compiler/Clang.cmake

@@ -23,4 +23,5 @@ include(Compiler/GNU)
 macro(__compiler_clang lang)
   __compiler_gnu(${lang})
   set(CMAKE_${lang}_COMPILE_OPTIONS_PIE "-fPIE")
+  set(CMAKE_INCLUDE_SYSTEM_FLAG_${lang} "-isystem ")
 endmacro()