Allow user to extend .PRECIOUS target

Allow user to extend .PRECIOUS target

References:
<http://lists.freedesktop.org/archives/systemd-devel/2013-July/012155.html>
<http://lists.gnu.org/archive/html/automake/2013-07/msg00011.html>

* bin/automake.in: Adjust to ensure we handle '.PRECIOUS' the same way
we do for '.PHONY' and '.MAKE'.
* lib/Automake/Rule.pm: Likewise.
* t/precious.sh: New test.
* t/list-of-tests.mk: Add it.
* t/phony.sh: Enhance a little while at it.
* NEWS: Update.
* THANKS: Likewise.

Reported-by: Holger Hans Peter Freyther <holger@freyther.de>
Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>