Declare support for CMake versions through 3.12 for own build
CMake 3.12 introduced a ...<max>
syntax in the version given to
cmake_minimum_required
to automatically set policies to NEW up
to that version. Use it to avoid listing policies explicitly.
The syntax is compatible with older versions of CMake such that they use
the extended version string for the CMAKE_MINIMUM_REQUIRED_VERSION
variable (which we don't use) but otherwise ignore it.