yacc.c: fix broken union when api.value.type=union and %defines are used

yacc.c: fix broken union when api.value.type=union and %defines are used

Reported by Rich Wilson.

* data/c.m4 (b4_symbol_type_register): Append to b4_union_members,
not b4_user_union_members.
The latter invokes the former, but it is the former which is reinitialized
to empty by b4_value_type_setup_union.
* tests/types.at: Check it.

This reveals another bug, this time in the case of glr.c parsers.

* data/glr.c: Generate the header file before the implementation file,
to be sure that the setup is run before what depends on it.