* data/glr.c (YYSTACKEXPANDABLE): Don't define if already defined.

(yyuserMerge): Provide a default case if b4_mergers is empty.
* tests/cxx-type.at (_AT_TEST_GLR_CXXTYPES): Define YYSTACKEXPANDABLE.
* tests/glr-regression.at
(Improper handling of embedded actions and dollar(-N) in GLR parsers):
YYSTYPE is char *, not char const *, so that strcpy ($$, ...) works.

tests/cxx-type.at

@@ -38,6 +38,7 @@ $1
 ]m4_bmatch([$2], [stmtMerge],
 [ static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);])[
   #define YYINITDEPTH 10
+  #define YYSTACKEXPANDABLE 1
   static char *format (char const *, ...);
   struct YYLTYPE;
 #if YYPURE

tests/glr-regression.at

@@ -121,7 +121,7 @@ AT_DATA_GRAMMAR([glr-regr2a.y],
 /* Reported by S. Eken */
 
 %{
-  #define YYSTYPE char const *
+  #define YYSTYPE char *
   #define yyfalse 0
   #define yytrue 1