Make -Werror= with a meta warning an error

The previous behavior was to just enable the meta warning's warnings.
This is an error now because it doesn't make sense to do that, does it?

src/asm/warning.c

@@ -118,6 +118,9 @@ void processWarningFlag(char const *flag)
 		/* TODO: improve the matching performance? */
 		if (!strcmp(flag, warningFlags[id])) {
 			/* We got a match! */
+			if (setError)
+				errx(1, "Cannot make meta warning \"%s\" into an error", flag);
+
 			uint8_t const *ptr =
 					metaWarningCommands[id - NB_WARNINGS];