* 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.