After configure the file `yaml-cpp-config.cmake.in` the result ends up with
empty variables. (see also the discussion in #774).
Rework this file and the call to `configure_package_config_file` according the
cmake documentation
(https://cmake.org/cmake/help/v3.22/module/CMakePackageConfigHelpers.html?highlight=configure_package_config#command:configure_package_config_file)
to overcome this issue and allow a simple `find_package` after install.
As there was some discussion about the place where to install the
`yaml-cpp-config.cmake` file, e.g. #1055, factor out the install location into
an extra variable to make it easier changing this location in the future.
