Is the issue here the avoidance of using any library that has a non-free license? If so, the polygon clipping library used in vgl (see vgl/internals/gpc.h) looks like another one.
Just want to drop in here with a general remark about the vxl (or actually; vnl) design w.r.t. netlib and other 3rd party libraries:
* There has never been a "hard" dependency on netlib (or whatever) external code library. We have deliberately kept those 3rd party code in a separate code branch "v3p".
* vnl/algo is a "C++ compatibility layer" on top of this v3p stuff.
It should at least be transparent to all vxl code using vnl/algo when one of the underlying implementations changes. All such implementation stuff is only present in files core/vnl/algo/*.cxx and .txx (21 of them currently).
Search for "#include <vnl/algo/vnl_netlib.h>" to locate all dependencies.