Mac: Check for otool/install_name_tool through xcrun

Also correct and extend the error message.

Change-Id: Ic00298cbfe24c1986e03834684e40fa0c16ea3c9
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>