[maint] yacc.c: restore ansi-c compatibility

[Akim Demaille]

commit 2da399d15f8cc08ffaf196735802fa381a43566d
Author: Akim Demaille <akim.demai...@gmail.com>
Date:   Sat May 9 17:02:32 2020 +0200

    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.

diff --git a/data/skeletons/yacc.c b/data/skeletons/yacc.c
index a790f6fa..a9f15c8d 100644
--- a/data/skeletons/yacc.c
+++ b/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];
diff --git a/src/parse-gram.c b/src/parse-gram.c
index a021e1aa..212e95d5 100644
--- a/src/parse-gram.c
+++ b/src/parse-gram.c
@@ -2705,7 +2705,7 @@ 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];