Code review comment for ~mitya57/compiz:lp1882792

Alberts: I removed all uses of SKIP_UNITY_BUILD_INCLUSION except two:

- outputdevices_x11.cpp had some strange linker errors when using Unity build, see the commit message.
- gsettings_util.c, that file redefined CCS_LOG_DOMAIN so it needed to be built separately in order for log functions to work.