Don't use ALIAS for polyclipping

conan-io/conan#4430 (comment)

Author: jellespijker

include/libnest2d/backends/clipper/CMakeLists.txt

@@ -2,9 +2,10 @@ add_library(clipperBackend INTERFACE)
 
 find_package(polyclipping 6.1)
 if(NOT TARGET polyclipping::polyclipping)
+    message(STATUS "Using require_package to obtain Clipper")
     require_package(Clipper 6.1 REQUIRED)
-    add_library(polyclipping::polyclipping ALIAS Clipper::Clipper)
-    set_target_properties(Clipper::Clipper PROPERTIES IMPORTED_GLOBAL TRUE)
+    add_library(polyclipping::polyclipping INTERFACE IMPORTED)
+    target_link_libraries(polyclipping::polyclipping PUBLIC Clipper::Clipper)
 endif()
 target_link_libraries(clipperBackend INTERFACE polyclipping::polyclipping)