ObjC: added -m mechanism to choose different compilers

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
ObjC: added/improved naming and set of compiler modules

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
ObjC: make objc also recompile if the compiler itself was updated

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>