Have cmake_minimum_required before project
Have cmake_minimum_required before project

I had a strange behavior using non-Apple clang on
OSX (clang-3.8.1). Symptom: it doesn't respect CMAKE_CXX_STANDARD=14 and
doesn't pass compiler flag "-std=c++14".

I looked at an old CMake issue tracker[1] and found that an easy fix is
to put "cmake_minimum_required" first before adding "project". I checked
that this solved my problem. There is also a suggested CMake policy[2],
which can justify this change.

[1]: https://cmake.org/Bug/view.php?id=15943
[2]: https://gitlab.kitware.com/cmake/cmake/issues/15544

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