glr.cc: support syntax_error exceptions

Kindly requested by Аскар Сафин (Askar Safin).
http://lists.gnu.org/archive/html/bug-bison/2018-12/msg00033.html

* data/skeletons/glr.c (b4_glr_cc_if): New.
Use it.
(yygetToken): Catch syntax_errors.
* data/skeletons/glr.cc (YY_EXCEPTIONS): New.
* tests/c++.at: Check it.

tests/c++.at

@@ -1020,6 +1020,7 @@ AT_CLEANUP
 ])
 
 AT_TEST([%skeleton "lalr1.cc"])
+AT_TEST([%skeleton "glr.cc"])
 
 m4_popdef([AT_TEST])