Fix autoupdate: updating `AC_OUTPUT_COMMANDS' was failing because

Fix autoupdate: updating `AC_OUTPUT_COMMANDS' was failing because
it includes another AU defined macro.
* autoupdate.sh (input.m4:_au_defun): New macro.
(au.m4): Use it instead of inlining _au_enable/_au_disable
invocations in the definition of all these macros.
(input.m4:__au_enable, input.m4:__au_disable): New macros.
(input.m4:_au_enable, input.m4:_au_disable): Use them.
* tests/mktests.sh (update_exclude_list): Add `AC_OUTPUT'.
Running it twice in a configure.in is not valid.
Add `AC_LINK_FILES' since it requires arguments.
* tests/tools.m4 (autoupdating AC_LINK_FILES): New test.