Comment fallthrough case statements so compilation succeeds with GCC 7.

GCC 7 Added a warning for implicit fallthroughs in switch cases.
It's enabled by -Wextra, and treated as an error due to -Werror so
compilation fails on GCC 7 and higher.
See -Wimplicit-fallthrough in the GCC manual.

token.c

@@ -447,6 +447,7 @@ eatstring(int quotechar)
 		case '\n':
 			if (!newlines)
 				break;
+            /* fall through */
 		case EOF:
 			reread();
 			scanerror(T_NULL,