cmake polyclipping examples

keith2018/SoftGLRender third_party/assimp/code/CMakeLists.txt :924 

find_package(polyclipping CONFIG REQUIRED)

google/filament third_party/libassimp/code/CMakeLists.txt :883 

find_package(polyclipping CONFIG REQUIRED)

meemknight/ourCraft thirdparty/assimp-5.2.4/code/CMakeLists.txt :923 

find_package(polyclipping CONFIG REQUIRED)