AC_CONFIG_MACRO_DIRS: new macro, mostly for aclocal

AC_CONFIG_MACRO_DIRS: new macro, mostly for aclocal

Similar to AC_CONFIG_MACRO_DIR, but accepts more than one argument.
This will allow projects to use several m4 macro local dirs.  This is
especially important for projects that are used as nested subpackages
of larger projects.

See also:
<http://lists.gnu.org/archive/html/autoconf/2011-12/msg00037.html>
<http://lists.gnu.org/archive/html/automake-patches/2012-07/msg00010.html>

* lib/autoconf/general.m4 (AC_CONFIG_MACRO_DIRS): New.  Expands to the
empty anyway, since it is only meant to be traced by tools like aclocal
and autoreconf.
(AC_CONFIG_MACRO_DIR): Updated comments.
* doc/autoconf.texi (@node "Input"): Document AC_CONFIG_MACRO_DIRS as
preferred over AC_CONFIG_MACRO_DIR.
* NEWS: Update.

Suggested-by: Eric Blake <eblake@redhat.com>
Helped-by: Nick Bowler <nbowler@elliptictech.com>
Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Eric Blake <eblake@redhat.com>