Add constexpr_fatal macro for constexpr errors

We need a complicated way to report constepxr errors in older gcc
versions because gcc 5.3 didn't quite implement all of C++14 constexpr
properly.

Add a macro that performs the workaround solution on gcc < 6, but uses a
neater solution (including an error message) on other compilers.

src/coordinates.h

@@ -44,8 +44,7 @@ constexpr int map_squares_per( scale s )
         case scale::overmap:
             return OMAPX * map_squares_per( scale::overmap_terrain );
         default:
-            debugmsg( "Requested scale of %d", s );
-            abort();
+            constexpr_fatal( 0, "Requested scale of %d", s );
     }
 }
 
@@ -67,8 +66,7 @@ constexpr origin origin_from_scale( scale s )
         case scale::overmap:
             return origin::overmap;
         default:
-            debugmsg( "Requested origin for of %d", s );
-            abort();
+            constexpr_fatal( origin::abs, "Requested origin for scale %d", s );
     }
 }
 

src/debug.h

@@ -80,6 +80,20 @@ inline void realDebugmsg( const char *const filename, const char *const line,
                          std::forward<Args>( args )... ) );
 }
 
+// A fatal error for use in constexpr functions
+// This exists for compatibility reasons.  On gcc 5.3 we need a
+// different implementation that is messier.
+// Pass a placeholder return value to be used on gcc 5.3 (it won't
+// actually be returned, it's just needed for the type), and then
+// args as if to debugmsg for the remaining args.
+#if defined(__GNUC__) && __GNUC__ < 6
+#define constexpr_fatal(ret, ...) \
+    do { return false ? ( ret ) : ( abort(), ( ret ) ); } while(false)
+#else
+#define constexpr_fatal(ret, ...) \
+    do { debugmsg(__VA_ARGS__); abort(); } while(false)
+#endif
+
 /**
  * Used to generate game report information.
  */