WIP Added libtess library extracted from

https://cgit.freedesktop.org/mesa/glu/
The library is stable (it should be, since it is in use since 1994),
but it is not thread safe: Its callbacks do not receive any pointer
to a context.

src/libslic3r/CMakeLists.txt

@@ -187,6 +187,7 @@ target_link_libraries(libslic3r
     ${EXPAT_LIBRARIES}
     ${GLEW_LIBRARIES}
     ${PNG_LIBRARIES}
+    glu-libtess
     polypartition
     poly2tri
     qhull