- 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_artifacts.cpp

@@ -8,7 +8,6 @@
 #include "p_local.h"
 #include "p_spec.h"
 #include "p_lnspec.h"
-#include "p_enemy.h"
 #include "p_effect.h"
 #include "a_artifacts.h"
 #include "sbar.h"
@@ -19,6 +18,7 @@
 #include "a_morph.h"
 #include "g_level.h"
 #include "doomstat.h"
+#include "v_palette.h"
 
 static FRandom pr_torch ("Torch");