maint: Upgrade OpenPGM to 2.1.28~dfsg

Upgrade OpenPGM to version 2.1.28 for ZeroMQ 2.0.x. Now we also use the
DFSG-compliant upstream release tarball.
Signed-off-by: Martin Lucina <mato@kotelna.sk>

configure.in

@@ -260,7 +260,7 @@ fi
 #  PGM extension
 pgm_ext="no"
 
-pgm_basename="libpgm-2.1.26"
+pgm_basename="libpgm-2-1-28~dfsg"
 
 AC_SUBST(pgm_basename)