For AC_CONFIG_LINKS(X,X), do not clean X in a non-VPATH build.

For AC_CONFIG_LINKS(X,X), do not clean X in a non-VPATH build.

* automake.in (rewrite_inputs_into_dependencies): Do distribute
inputs where input and output name are equal.  This relies on
the assumption that it only happens with AC_CONFIG_LINKS.
(handle_configure) <CONFIG_CLEAN_VPATH_FILES>: New variable, for
links where source and dest are equal.
* lib/am/clean.am (distclean-generic): Remove, if build tree not
equal source tree.
* tests/conflnk4.test: New test.
* tests/Makefile.am: Adjust.
* NEWS: Update.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>