Generated code triggers a warning

[Csaba Raduly]

Line 2687 in parse-gram.c reads:
yyerror (yymsg);

This triggers a warning from gcc under -Wformat=2
"format not a string literal and no format arguments"

The line should be 
yyerror ("%s", yymsg);

(affects both 2.3 and 2.4)

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