autoconf: update m4/pkg.m4

autoconf: update m4/pkg.m4

The provenance of the m4 pkg.m4 macro library was not documented
and it was not clear how and if we can update this without breaking
things. Document where this comes from, update us to the latest
release from git and put a warning about avoiding private evolutions
so that instead things go upstream.

The pkg-config m4 macro library comes from the upstream pkg-config
git repository [0]. If you update the m4 library update this file
with provenance and last commit ID information. For documentation
on how to use this read the pkg-config(1) man page.

Tree: git://anongit.freedesktop.org/pkg-config

The last synch was from commit:

commit 4f0084d9d3b8726a797a373c7ec5e406000995d0
Author: Dan Nicholson <dbn.lists@gmail.com>
Date:   Mon Feb 3 15:59:18 2014 -0800

    glib: Fix Makefiles to suppress warnings from automake

    With the newly added glib.mk, some of the noinst_* variables need to use
    += in the evaluation to avoid multiple definition warnings from
    automake.

Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Tim Deegan <tim@xen.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
[ ijc -- ran autogen.sh ]