devel/gpds: Update to 1.8.4

[jbo]

Summary

Changelog: https://github.com/simulton/gpds/releases/tag/1.8.4

[jbo]

Two "major" things are happening here:

• Upstream supports consuming the 3rd-party tinxymxl2 dependency from the system (via CMake's find_package() since 1.8.3)
• spdlogis no longer a dependency of this library. Consumers using the spdlog feature are expected to link towards it themselves.

[tcberner]