Fix Makefile for OSX

Bump base SDK from 10.5 -> 10.7
Just use the builtin __APPLE__

code/sys/sys_osx.m

@@ -20,7 +20,7 @@ Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 ===========================================================================
 */
 
-#ifndef MACOS_X
+#ifndef __APPLE__
 #error This file is for Mac OS X only. You probably should not compile it.
 #endif