make: Workaround limitation in Naia regarding targets

Naia's Makefile.wmltools does not currently support us not needing any
specific configuration sets besides the __WMLUNITS__ set, so without a
dummy configuration set it's impossible to make any configuration
set-dependent targets with it (test, stats).

(CC #76)

Makefile

@@ -8,9 +8,7 @@ EXTRASYMS            := ENABLE_DWARVISH_RUNESMITH,ENABLE_DWARVISH_ARCANISTER
 DIST_VERSION_FILE    := dist/VERSION
 
 DIFFICULTIES         := EASY NORMAL HARD
-PACKS                := \
-	CAMPAIGN_INVASION_FROM_THE_UNKNOWN_EPISODE_I \
-	CAMPAIGN_INVASION_FROM_THE_UNKNOWN_EPISODE_II
+PACKS                := __DUMMY_CONFIG_SET__
 
 SCENARIO_DIRS        := scenarios