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.

THANKS

@@ -112,6 +112,7 @@ Quoc Peyrot               chojin@lrde.epita.fr
 R Blake                   blakers@mac.com
 Raja R Harinath           harinath@cs.umn.edu
 Ralf Wildenhues           Ralf.Wildenhues@gmx.de
+Rich Wilson               richaw@gmail.com
 Richard Stallman          rms@gnu.org
 Rici Lake                 ricilake@gmail.com
 Rob Conde                 rob.conde@ai-solutions.com