- 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_doom/a_keen.cpp

@@ -1,3 +1,4 @@
+/*
 #include "actor.h"
 #include "info.h"
 #include "p_local.h"
@@ -5,6 +6,7 @@
 #include "p_enemy.h"
 #include "a_action.h"
 #include "thingdef/thingdef.h"
+*/
 
 //
 // A_KeenDie