Fix: remove the CMAKE_OSX_DEPLOYMENT_TARGET auto detect by CMake

[ErniGH]

If we don't initialize the variable CMAKE_OSX_DEPLOYMENT_TARGET, it tries to retrieve it from an environment variable, and if not, it calculates it based on the host platform. To prevent this, we check that it is initialized before the call to project().

https://cmake.org/cmake/help/latest/variable/CMAKE_OSX_DEPLOYMENT_TARGET.html

Close: #627

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