* automake.in (%silent_variable_override): New variable.

* automake.in (%silent_variable_override): New variable.
(macro_define): Warn about variable definitions overriding
Automake variables.
(rule_define): Warn about target definitions overriding
Automake variables.  Fix $condmsg definition.
* automake.texi (Invoking Automake): Document the `override'
category.
* lib/Automake/ChannelDefs.pm (usage): Likewise.
* lib/am/texinfos.am [!%?CYGNUS] (MAKEINFO): Do not define,
this is already done in m4/init.m4.
* tests/dejagnu2.test: Run $MAKE, don't only grep.  Use
-Wno-override, and make sure we get a warning without.
* tests/exeext2.test, tests/substtarg.test: Use -Wno-override,
and make sure we get a warning without.
* tests/exeext3.test, tests/java2.test, tests/nolink.test,
tests/subpkg.test, tests/vartest.test: Use -Wno-override.
* tests/txinfo13.test: Use installcheck-local instead of overriding
installcheck.
* tests/txinfo2.test: Run $MAKE on the real Makefile (this test
used to succeed thanks to a failure...)
* tests/location.test: Adjust expected messages.
* tests/Makefile.am (XFAIL_TESTS): Add txinfo5.test.