build: restore C90 compatibility

* src/parse-gram.y, src/parse-gram.c: Don't use // comments.

src/parse-gram.y

@@ -46,7 +46,7 @@ static void version_check (location const *loc, char const *version);
 
 static void gram_error (location const *, char const *);
 
-/// A string that describes a char (e.g., 'a' -> "'a'").
+/* A string that describes a char (e.g., 'a' -> "'a'").  */
 static char const *char_name (char);
 %}