buildcfg: include correct demos for each iwad

It's a bit hard to see given the flat format of buildcfg.txt but we need
to do this:

if doom2
  include demo{1,2,3}_d2
else
  include demo1_ud // both shareware and ultdoom
  if ultdoom
    include demo{2,3,4}_ud
  endif
endif

The previous code had doom2 and ultdoom separately and always included
an unsourced demo2 and demo3, which was breaking the ultdoom IWAD.

Signed-off-by: RjY <rjy@users.sourceforge.net>
Signed-off-by: Mike Swanson <mikeonthecomputer@gmail.com>

buildcfg.txt

@@ -196,24 +196,20 @@ DEMO1 = demo1_d2
 DEMO2 = demo2_d2
 DEMO3 = demo3_d2
 
-#else
+#else /* not DOOM2 */
 
-DEMO1
-
-#endif /* #ifdef DOOM2 */
-
-DEMO2
-DEMO3
+DEMO1 = demo1_ud
 
 #ifdef ULTDOOM
 
-DEMO1 = demo1_ud
 DEMO2 = demo2_ud
 DEMO3 = demo3_ud
 DEMO4 = demo4_ud
 
 #endif /* #ifdef ULTDOOM */
 
+#endif /* #ifdef DOOM2 */
+
 #ifdef FREEDM
 
 FREEDM