build: be sure to include config.h first in the generated parser

Using %code for config.h is wrong, as some headers will already have
been included by Bison.  In some cases, e.g., glibc's string.h, this
results in some declaration not being made for lack of definition of
_GNU_SOURCE, which is performed by config.h.

* src/parse-gram.y: here.

src/parse-gram.y

@@ -23,9 +23,14 @@
   #include "symtab.h"
 }
 
+%code top
+{
+  /* On column 0 to please syntax-check.  */
+#include <config.h>
+}
+
 %code
 {
-  #include <config.h>
   #include "system.h"
 
   #include "c-ctype.h"