- Changed compilation for g_doom, g_heretic, g_hexen and g_strife folders

so that all files are included by a central one instead of compiling 
  each one separately. This speeds up the compilation process by 25%
  when doing a complete rebuild in Visual C.
- Cleaned up more header dependencies.

SVN r1226 (trunk)

src/g_shared/a_morph.cpp

@@ -4,7 +4,6 @@
 #include "gstrings.h"
 #include "p_local.h"
 #include "gi.h"
-#include "p_enemy.h"
 #include "s_sound.h"
 #include "m_random.h"
 #include "a_sharedglobal.h"