maint: restore ANSI 89 compliance

* data/bison.m4, src/conflicts.c, src/files.c, src/output.c,
* src/symtab.c: Use /* ... */ comments only.
Declare variables before statements.

src/files.c

@@ -140,8 +140,8 @@ xfdopen (int fd, char const *mode)
   FILE *res = fdopen (fd, mode);
   if (! res)
     error (EXIT_FAILURE, get_errno (),
-           // On a separate line to please the "unmarked_diagnostics"
-           // syntax-check.
+           /* On a separate line to please the "unmarked_diagnostics"
+              syntax-check. */
            "fdopen");
   return res;
 }