Build: Include headers from LibC, LibM, and LibPthread with -isystem

Make sure that userspace is always referencing "system" headers in a way
that would build on target :). This means removing the explicit
include_directories of Libraries/LibC in favor of having it export its
headers as SYSTEM. Also remove a redundant include_directories of
Libraries in the 'serenity build' part of the build script. It's already
set at the top.

This causes issues for the Kernel, and for crt0.o. These special cases
are handled individually.

Libraries/LibM/math.cpp

@@ -24,8 +24,8 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <LibC/assert.h>
-#include <LibM/math.h>
+#include <assert.h>
+#include <math.h>
 #include <stdint.h>
 #include <stdlib.h>