Correct LCG Setup for Downstream Projects, main branch (2025.01.15.)

Attila Krasznahorkayrequested to merge
akraszna/athena:CorrectLCGSetupForDownstream-main-20250115 into main

Made AthAnalysis and AthGeneration clients find Boost correctly.

As I found while investigating @ravinab's report in !77064 (comment 8930347), even after !77064 (merged) and !77095 (merged), the Boost changes described in ATLINFR-5571 are still not working correctly with the AthAnalysis and AthGeneration nightlies. 😭

These were the two remaining LCG based projects that didn't set up LCG as part of their own CMake configuration, but rather left it up to AthAnalysisExternals and AthGenerationExternals. (It's a bit of a longer story.) Since I only ever tested by previous updates on top of Athena, I didn't find these issues earlier. 😦

@ravinab, hopefully this will finally fix things for you. 🤞

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