yacc.c: restore ansi-c compatibility

Reported by neok-m4700.
https://github.com/akimd/bison/issues/37

* data/skeletons/yacc.c: Don't use // comments.

data/skeletons/yacc.c

@@ -1979,7 +1979,7 @@ yyerrorlab:
 yyerrlab1:
   yyerrstatus = 3;      /* Each real token shifted decrements this.  */
 
-  // Pop stack until we find a state that shifts the error token.
+  /* Pop stack until we find a state that shifts the error token.  */
   for (;;)
     {
       yyn = yypact[yystate];