- adding new build flag options (NO_LIB, NO_CUDA...)

- move python build section into the python directory (cleaning up)

- fix some broken dependencies

- remove the public_headers targets if doxygen was not found

TODO :
- fix MSVC targets for public headers (wbn if MSVC didn't require the pro version
  in order to support solution folders)
- fix osd_regression to not build if -DNO_LIB is present (ie. fix the broken dependency)