Generate and install a pkg-config file. (#142)

Generate and install a pkg-config file. (#142)

* Generate and install a pkg-config file.

* Use Makefile libdir and includedir for pkg-config.

This splits the prefix out from these variables, so that
the same variables can be used to construct the pkg-config
file.

* Update the manifest for installation of pkg-config file.

* Revert "Use Makefile libdir and includedir for pkg-config."

This reverts commit a4cd6dc64a357dd5b91781045b3ac1c4679af97f.

* Use Makefile libdir and includedir for pkg-config.

This splits the prefix out from these variables, so that
the same variables can be used to construct the pkg-config
file.