CMake build system does not install pkg-config files

[madebr]

Comparing the build prefix of a autools and CMake build, there are a few files missing in a CMake install:

• lib/pkgconfig/opusfile.pc
• lib/pkgconfig/opusurl.pc
• share/doc/opusfile/AUTHORS
• share/doc/opusfile/COPYING
• share/doc/opusfile/README.md

This is annoying because my Linux distro (=Fedora) currently uses autotools.
I can ask them to switch to the CMake build but since this might cause build breakage in other packages, it's unlikely they will honor this request.

So I propose to install these files in the CMake build as well.

I am willing to create a pr for this, if a maintainer is willing to review them in a timely matter. (Please ack this issue if doing so)

Thanks.

[madebr]

#35 is apparently trying to add this support.