The cmake/spdlogConfig.cmake.in file properly takes into account the fmt
package dependency when building with SPDLOG_FMT_EXTERNAL:BOOL=ON but
not when built with SPDLOG_FMT_EXTERNAL_HO:BOOL=ON.
Prior to these changes SPDLOG_FMT_EXTERNAL_HO:BOOL=ON results in
exported targets with INTERFACE_LINK_LIBRARIES that contain
fmt::fmt-header-only.
As such, the installed spdlogConfig.cmake file should attempt to find
that dependency for the consumer.
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