Rules are added to libs.mk to generate a vpx.pc, which is
installed as pkgconfig/vpx.pc under the target library directory.
This also requires the install path prefix be exported directly
in config.mk.

Some systems use a tool called pkg-config to query information
about intalled libraries or other resources, based on database
files provided by the packages themselves at install time.

Providing such a file for libvpx simplifies integration with
other build systems, and provides an easy avenue for developers
to test against their own builds of the library.

Change-Id: I4e32a8fbb53fc331aa95eb207c63dd70a76d18ed