Fix for PR automake/289:

Fix for PR automake/289:
* automake.in (Automake::Struct::libtool_tag): New attribute.  Define
it for the language that have a Libtool tag.
(%libtool_tags): New variable.
(handle_languages, define_compiler_variable)
(define_linker_variable): Pass --tag=XXX to libtool if supported.
(scan_autoconf_traces): Scan for _LT_AC_TAGCONFIG and AC_LIBTOOL_TAGS.
* tests/libtool3.test, tests/subobj9.test: Check that --tag=XXX is
output.