Visual Studio builds: Fix .pc generation
Visual Studio builds: Fix .pc generation

The previous update did not account for when no exec_prefix is spcified,
so update that, and use ${prefix} by default.  Clean up a bit as well.

johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib