Add a new CMake configuration type: Optimized

Now Root can be built with the following build types:
o MinSizeRel
o Debug
o RelWithDebInfo
o Release
o Optimized
selecting via the CMake variable CMAKE_BUILD_TYPE.
The optimised builds are characterised by aggressive compiler
optimisations, namely "-Ofast" and "-O3 -ffast-math" for gcc and
clang respectively.
Roottest runs fine (and faster: some 10%).