build: Point missing resources at dummy resources.

Currently the wadinfo-builder script does this automatically, but
there are so few missing resources now that it's going to be simpler
to just get rid of it. These are the few remaining resources which
don't exist in the Freedoom tree.

Part of #485.

buildcfg.txt

@@ -50,7 +50,7 @@ E3M1 = c3m1
 E3M2 = c3m2
 E3M3 = c3m3
 E3M4 = c3m4
-E3M5 = c3m5
+E3M5 = dummy ; c3m5
 E3M6 = c3m6
 E3M7 = c3m7
 E3M8 = c3m8
@@ -129,7 +129,7 @@ MAP22
 MAP23
 MAP24
 MAP25
-MAP26
+MAP26 = dummy
 MAP27
 MAP28
 MAP29
@@ -302,13 +302,13 @@ DSSKESWG
 DSBSPDTH
 DSVILDTH
 DSKNTDTH
-DSPEDTH
+DSPEDTH  = dummy
 DSSKEDTH
 DSBSPACT
-DSBSPWLK
+DSBSPWLK = dummy
 DSVILACT
-DSFLAME
-DSFLAMST
+DSFLAME  = dummy
+DSFLAMST = dummy
 DSBOSPIT
 DSBOSCUB
 DSBOSSIT