Bump minimum required CMake version
Bump minimum required CMake version

Bump minimum required CMake version to 2.8.12 now that it's released.

That will also avoid the following deprecation warning:

CMake Deprecation Warning at .../civetweb/CMakeLists.txt:2 (cmake_minimum_required):
  Compatibility with CMake < 2.8.12 will be removed from a future version of
  CMake.

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