CMake package config (#412)

* Use variables from GNUInstallDirs everywhere

* Componentize the install rules

* Add headers to the interface include path

* Add install rules for CMake package config

* Associate inline headers to a target

Previously inline_headers was a UTILITY target, which cannot be
installed, so it was renamed to inline_headers_util and a new
inline_headers INTERFACE library target is introduced instead.

Co-authored-by: friendlyanon <friendlyanon@users.noreply.github.com>