- 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/thingdef/thingdef.cpp

@@ -51,13 +51,11 @@
 #include "s_sound.h"
 #include "cmdlib.h"
 #include "p_lnspec.h"
-#include "p_enemy.h"
 #include "a_action.h"
 #include "decallib.h"
 #include "m_random.h"
 #include "i_system.h"
 #include "p_local.h"
-#include "p_effect.h"
 #include "v_palette.h"
 #include "doomerrors.h"
 #include "a_hexenglobal.h"