Replace last occurrences of `include` install paths

Replace last occurrences of `include` install paths

* 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.