add cmake options to enable/disable optional dependencies

This commit adds
- WITH_LIBUSB
- WITH_OPENNI
- WITH_FZAPI
- WITH_PXCAPI
- WITH_PNG
- WITH_QHULL
- WITH_CUDA
- WITH_QT
- WITH_VTK
- WITH_PCAP
- WITH_DOCS (doxygen documentation)
- WITH_TUTORIALS (Sphinx tutorial documentation)

Phew, this project has quite a number of optional dependencies..

With this change it's possible to build pcl without optional dependencies even
though they are available on the system.