possible to build with or without -Duse_xyz; initial support for Paths

If -Duse_xyz is omitted from the compliler directives, Z-supporting
code will mostly be left out, and Z-related tests will be skipped.
   and
Clipper 6 introduces Paths to replace Polygons. Paths may be open
or closed. This commit allows open path input with the new
add_*_polyline() methods. Output forthcoming.