buildcfg: Separate demos to specific IWAD targets

The same demos won't work across IWADs; the previous commit completely
locked up Boom when using doom.wad.  Now there will be independent demos
for each IWAD.

Signed-off-by: Mike Swanson <mikeonthecomputer@gmail.com>

buildcfg.txt

@@ -185,14 +185,30 @@ MAP32
 PLAYPAL
 COLORMAP
 ENDOOM
-DEMO1
-DEMO2
-DEMO3
 GENMIDI
 ;DMXGUS
 ;DMXGUSC
 FREEDOOM
 
+#ifdef DOOM2
+
+DEMO1 = demo1_d2
+
+#else
+
+DEMO1
+
+#endif /* #ifdef DOOM2 */
+
+DEMO2
+DEMO3
+
+#ifdef ULTDOOM
+
+DEMO4
+
+#endif /* #ifdef ULTDOOM */
+
 ; boom colormaps
 
 C_START