Deprecate obsolete macro AM_WITH_REGEX.

Deprecate obsolete macro AM_WITH_REGEX.

* m4/regex.m4: Document the `AM_WITH_REGEX' macro as obsolete,
and state that it should be removed two years from now.
(AM_WITH_REGEX): Raise an m4-time warning of the "obsolete"
category when this macro is used.
* doc/automake.texi (Public Macros): Move description of
`AM_WITH_REGEX' from here ...
(Obsolete Macros): ... to here, and declare it as obsolete
and "to be removed in a future version".
* tests/regex-obsolete.test: New test.
* tests/Makefile.am (TESTS): Update.