suzerain: add pkg.m4

Per renewed discussion on the autoconf mailing list,
https://lists.gnu.org/archive/html/autoconf/2011-07/msg00029.html,
adding pkg.m4 to the distribution so that it can be found
during re-conf even if pkg-config is not installed.  Provides
better error messages to folks.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@43468 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: #3022 radialflow.m rewrite

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@43469 0bd85f0b-c151-43b9-a1e9-27231104551f