This resolves an irritating issue in the code editor where SDL2-specific functions could't be found. Not entirely sure about the SDL include file logic for other operating systems than Linux so the #ifdef directives may need to be updated further at a later date.
Also a general cleanup of CMakeLists.txt and many small code changes to satisfy Clang that complained quite a lot and actually wouldn't compile the binary at all at first.
CMAKE_INSTALL_PREFIX and CMAKE_INSTALL_DATAROOTDIR are now used to resolve the resource path. As of this commit, there are only two paths where resources are searched, under the user home directory and under this install prefix directory (which defaults to /usr/local/share/emulationstation/resources but can be set to for instance /opt/share/emulationstation/resources using the appropriate CMake flags).
