[ecbadeaux]

Link Threads::Threads wherever Boost.Asio is used

Summary

Fix undefined pthread_* link errors in the udp_server and uds_server test helpers on toolchains where the thread library is a separate libpthread.

Problem

#139 changed several targets from Boost::system to the header-only Boost::boost. Boost::system was what pulled -pthread into the link line. After the change, udp_server and uds_server link no thread library at all, even though Boost.Asio needs one. The link fails on an older Linux toolchain with:

undefined reference to `pthread_create'
undefined reference to `pthread_join'
undefined reference to `pthread_detach'
undefined reference to `pthread_sigmask'
undefined reference to `pthread_condattr_setclock'

I found this when the atlas-system-agent package build (which builds spectator-cpp via add_subdirectory) failed after moving to v2.4.1.

Changes

• CMakeLists.txt: add find_package(Threads REQUIRED).
• udp_server and uds_server executables: link Threads::Threads. This is the fix for the errors above.
• udp_server_lib and uds_server_lib: replace the bare pthread with Threads::Threads.
• spectator-writer-types and spectator-registry: link Threads::Threads publicly, so consumers of the libraries get the thread flag instead of relying on a transitive dependency. This guards against the same failure in other targets.

Follow-up

Tag v2.4.2 after merging, so atlas-system-agent can repin to it.