As per CMake's Importing and Exporting Guide[1],
configure_package_config_file() should be used for configuring the
package configuration file, not the regular configure_file() function.
This ensures that a spdlog package built on one system (with a given
directory tree) can be imported from a different system -
e.g. creating a pre-compiled spdlog package for use on different
systems.
[1]: https://cmake.org/cmake/help/git-stage/guide/importing-exporting/index.html#id8
