- 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/p_user.cpp

@@ -35,7 +35,6 @@
 #include "m_random.h"
 #include "p_pspr.h"
 #include "p_enemy.h"
-#include "p_effect.h"
 #include "s_sound.h"
 #include "a_sharedglobal.h"
 #include "a_keys.h"