CMake generate pkg-config .pc

CMake can generate pkg-config .pc files for packages. The .pc file can be used by 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.