glr2.cc: add support for parse.assert

* data/skeletons/glr2.cc: Fake support of parse.assert, so that the
tests can use it without failing.

TODO

@@ -198,6 +198,10 @@ yycharp is no longer a Pointer.  And yystackp should probably also be a referenc
 Don't use YYSTYPE and YYLTYPE but parser::semantic_type and
 parser::location_type.  Undefine YYSTYPE and YYLTYPE.
 
+*** parse.assert
+Currently all the assertions are enabled.  Once we are confident in glr2.cc,
+let parse.assert use the same approach as in lalr1.cc.
+
 *** glr.c
 When glr2.cc fully replaces glr.cc, get rid of the glr.cc scaffolding in
 glr.c.

data/skeletons/glr2.cc

@@ -456,7 +456,6 @@ static YYLTYPE yyloc_default][]b4_yyloc_default;])[
 [#include <cstdio>
 #include <cstdlib>
 
-
 #ifndef YY_
 # if defined YYENABLE_NLS && YYENABLE_NLS
 #  if ENABLE_NLS
@@ -504,6 +503,8 @@ static YYLTYPE yyloc_default][]b4_yyloc_default;])[
 ]b4_null_define[
 ]b4_cast_define[
 
+// FIXME: Use the same conventions as lalr1.cc.
+]b4_parse_assert_if[
 #ifndef YYASSERT
 # define YYASSERT(Condition) ((void) ((Condition) || (abort (), 0)))
 #endif