Does not compile on MacOSX 10.13 with clang 9.1.0

Thanks to Oliver Fischer for trying to compile on his laptop. It fails with this message:

In file included from /Users/oliver/Corsika/corsika-master/Processes/TrackingLine/TrackingLine.cc:17: /Users/oliver/Corsika/corsika_build/include/corsika/process/tracking_line/TrackingLine.h:21:10: fatal error: 'optional' file not found

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