cmake: Update the minimum required version to 2.8.12 (except Windows)

cmake: Update the minimum required version to 2.8.12 (except Windows)

Windows minimum required is already 3.12.

This change avoids this warning:
CMake Deprecation Warning at CMakeLists.txt:7 (cmake_minimum_required):
  Compatibility with CMake < 2.8.12 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.

(Same as in libpcap.)