* lib/Automake/Variable.pm (define): Rework the way we check

* lib/Automake/Variable.pm (define): Rework the way we check
Automake variable definition.  Ensure consistent :=/+=/=
definitions only for variables that have been and are defined by
Automake, and make it an internal error.  Ignore Automake
attempts to touch a user variable, even with += assignments.
* tests/txinfo29.test: New file.
* tests/Makefile.am (TESTS): Add txinfo29.test.
Reported by Bruce Korb.