CC to Makefile. Before compiling, skim through the Makefile. Standard is cc. In the tar file you can find the test program for the nclip library. Reads two clockwise oriented polygons from stdin, and clips those.
Results comes on stdout.
The actual clip routine. Only file in the nclip library which is decent documented. The rest of the nclip library. Not of real interest, but needed to get nclip running. Not likely to have big errors.