CMake deprecation warning

[mlund]

On newer versions of cmake I get this warning:

CMake Deprecation Warning at CMakeLists.txt:9 (cmake_minimum_required):
  Compatibility with CMake < 3.5 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.

[ebenali]

probably just copy the header into your tree under 3rdparty/ or whatever

[mlund]

probably just copy the header into your tree under 3rdparty/ or whatever

I can certainly do that, but the warning suggests that the current CMakeLists.txt will become outdated. An alternative solution would be to increase the minimum CMake version to 3.5, released in 2016.

[martinRenou]

Fixed by #65