[cmake] Drop the support of CMake 2.x.

* CMakeLists.txt: Require CMake 3.0 (released on 2014) or newer.

The issue #1059 reports the difficulty to support both of
CMake 2.x and newer one by single CMakeLists.txt without
the inflation of cmake_policy() workarounds.

For better maintainability, the support of CMake 2.x is
dropped.