PThread Usage Fix, master branch (2022.02.17.)

Added explicitly settings for THREADS_PREFER_PTHREAD_FLAG in each project's exported configuration. This way making sure that by the first time that find_package(Threads) is called in a project using one of these (like WorkDir), FindThreads.cmake would work as intended.

This is meant to (largely) fix the issue described in ATLINFR-4377, and is a step-sibling of atlasexternals!928 (merged).

@fwinkl, @tadej, I made some tests with a full Athena build, just to see how "reproducible" its configuration is. Building the Athena project against atlasexternals!928 (merged) with -DCMAKE_EXPORT_COMPILE_COMMANDS=TRUE to generate a compile_commands.json file, I compared the generated files made "in the first" and "in the second" configuration. To my great relief the commands didn't differ a whole lot. But they were also not exactly the same. 😦

I found that the configuration of AthenaPoolTestData and code depending on it, changed all so very slightly. That one is coming from CLHEP, in a quite elaborate way. I'll open a separate ticket about it, but luckily it only affects the build of 32 source files in the entire Athena project. So the CI should not be terribly concerned about that luckily...