CMake: Build directory spdlogConfigTargets.cmake doesn't provide a spdlog::spdlog_header_only target

[robertmaynard]

The installed version of spdlogConfigTargets.cmake includes both the spdlog::spdlog and spdlog::spdlog_header_only targets. The build directory version only provides the compiled library target, which causes issues for dowstream consumers of the header only version.

This is caused by a mismatch between what targets are exported via the install and export commands.
Updating line https:/gabime/spdlog/blob/v1.x/CMakeLists.txt#L339 to also include spdlog_header_only would fix this issue.

[robertmaynard]

In addition by not exporting the spdlog_header_only target it means that any downstream user that uses spdlog from source can't export a dependency that has a public dependency on spdlog_header_only

  export called with target "use_spdlog" which requires target "spdlog_header_only"
  that is not in any export set.

[gabime]

@robertmaynard Thanks for reporting. PR to fix this would be most welcome.

[gabime]

@robertmaynard Any chance you send PR for this before next release (few days)?