Upgrade to OCCT 6.8.0

Remove OCE_DISABLE_BSPLINE_MESHER option: BRepMesh_FastDiscretFace had been
deeply modified in OCCTT 6.8.0, and keeping this patch around was not trivial.
Since we are not sure anymore that this option is really needed, it is now
removed.  Closes: #277.

New option: OCE_WITH_VTK, OFF by default.

Clean up oce-config.h, many checks and cpp symbols have been removed in
OCCT 6.8.0.