Tell GCC/Clang to build in C++11 mode

The override
specifier is a C++11 feature.

Some compilers (like GCC and Clang) build in C++03 mode by default, and
you must explicitly tell them to build in C++11 mode.

The easiest way to do this is to use CMAKE_CXX_COMPILER_ID for detecting
the compiler, and add_compile_options for passing the correct
options to it.

These features require CMake 2.8.12, so we set the minimum required CMake
version to it.