Generate a vpx.pc file for pkg-config.
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