Silence various warnings in builds removing content

The MAKEANDPLAY, NO_CUSTOM_LEVELS, and NO_EDITOR defines remove content
or features. However, they then raise several warnings because of some
cases, functions, or variables that end up not being used.

This silences them by using the UNUSED macro, or by adding a default
catch-all case if the define is defined (so unhandled cases will still
raise warnings in a build that doesn't have these defines).

desktop_version/src/Script.cpp

@@ -19,6 +19,7 @@
 #include "Map.h"
 #include "Music.h"
 #include "Unreachable.h"
+#include "Unused.h"
 #include "UtilityClass.h"
 #include "VFormat.h"
 #include "Vlogging.h"
@@ -2741,7 +2742,9 @@ void scriptclass::startgamemode(const enum StartMode mode)
         }
         break;
 
-#ifndef NO_CUSTOM_LEVELS
+#ifdef NO_CUSTOM_LEVELS
+        UNUSED(gotoerrorloadinglevel);
+#else
 # ifndef NO_EDITOR
     case Start_EDITOR:
         cl.reset();
@@ -2836,6 +2839,12 @@ void scriptclass::startgamemode(const enum StartMode mode)
 
     case Start_QUIT:
         VVV_unreachable();
+
+#if defined(NO_CUSTOM_LEVELS) || defined(NO_EDITOR)
+    /* Silence warnings about unhandled cases. */
+    default:
+        break;
+#endif
     }
 
     game.gravitycontrol = game.savegc;