RadX uses the system libraries when possible.

This edits the RadX provision script so that the system netcdf, hdf5, and udunits are used when compiling RadX.   RadX still adds significantly to the size of the VM even with this change as the binaries and libraries are quite large (change to last line to `sudo ./build_radx /tmp/radx` and look in the /tmp/radx directory for details).