maint: address syntax-check errors.

* cfg.mk: Ignore the "error" call in tests/c++.at, it is not to be
translated.
* doc/bison.texi: Fix incorrect @pxref use.
* po/POTFILES.in: Add missing file.
* src/print_graph.c: Remove useless include.

src/print_graph.c

@@ -18,7 +18,6 @@
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #include <config.h>
-#include <quotearg.h>
 #include "system.h"
 
 #include "LR0.h"