Support std::experimental::filesystem

[mabraham]

For example, SLES 15 uses gcc 7.5 as the system compiler and C++ standard library. This supports C++17, but does so for std::filesystem via the std::experimental namespace and requires linking an extra library and including a specially named header.

This change imports BSD-licensed community CMake content to make building unitrace smoother on such platforms.

[mabraham]

Tested on LRZ cluster

[zma2]

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