* Distributions might need to change the header dir
* Also change the template paths in the `.pc` files to
the idiomatic CMake `GNUInstallDirs` full paths,
which are always correct, regardless of whether the
user specified relative or absolute paths. This makes
the build system somewhat easier and more idiomatic.
The prefix and libdir lines at the top of the .pc files are variable
declarations. Set these variables appropriately, reference them in the
rest of the file, and prevent cmake from expanding ${}-style references.
Switch back to @prefix@ and @libdir@ for compatibility with autoconf.
Make the descriptions consistent and update the URLs.
Aisha Tammy