Added 'MOAI_ROOT' variable.

All source files should be specified relative to this variable as opposed to having paths relative to the build scripts themselves.
Updated source paths to be relative to MOAI_ROOT variable
Changed relative source paths to be relative to MOAI_ROOT
Fixed OPENGL_LIBRARIES variable for Linux builds
Fixed typo in host-test cmake build script
Changed more relative source paths for cmake build scripts to be relative to MOAI_ROOT