CMake generate pkg-config .pc

CMake can generate pkg-config .pc files for packages. The .pc file serves as a Rosetta stone for many build systems. While we normally use the .cmake files for our packages, we also include the .pc files for non-CMake users. Meson has a built-in function for generating pkg-config files, but CMake does not yet.

A good basic reference for pkg-config .pc syntax is helpful. We use a template with contents generated by CMake configure_file() and associated variables.