* 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.