configure.ac: Check for pkg-config presence

If pkg-config is not installed, a very cryptic error occurs when
autogen.sh runs the configure script.  Add a check from:

http://lists.gnu.org/archive/html/automake/2011-03/msg00008.html

to check for pkg-config presence and if pkg-config is not found, a stub
macro disables any configuration of any feature relying on its presence.