Bring pkg.m4 in from pkgconfig version 0.28 so we can use the
PKG_PROG_PKG_CONFIG macro to try searching for pkg-config even when
it isn't installed.  Without this (and without pkg-config installed)
the configure script is broken.  With it, it gives a message saying
pkg-config isn't found.

Change-Id: I4c5af4cedd8c1927c1e00dd6b42faef3f31b84d4
Reviewed-on: https://code.wireshark.org/review/5671
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Anders Broman <a.broman58@gmail.com>