CMake: Require CMake 3.0
Even [Debian OldStable][] supports the latest major version of CMake.
[Debian OldStable]:
-cmake_minimum_required (VERSION 2.8.8)
+cmake_minimum_required (VERSION 3.0)
# allows us to override platform specific variables
set(CMAKE_USER_MAKE_RULES_OVERRIDE "${CMAKE_SOURCE_DIR}/cmake/Platform.cmake")
