Random movement gives the same path for all entities on Mac OS

Created by: christopho

See https://bitbucket.org/zeldaforce/zelda-lafd/issues/31/vegas-movement or https://youtu.be/JjwY2QJeIzY?t=12m34s

It looks like the random numbers returned by Random::get_number() are the same when the function is called in the same second. Maybe the thread_local storage specifier is not working on Mac? @morwenn, any clue?

@vlag, can you check if replacing thread_local by static at lines 78 and 79 of src/lowlevel/Random.cpp solves the problem?

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