check that policy CMP0087 exists in python/CMakeLists.txt file and set it to new if available

python/CMakeLists.txt

@@ -6,6 +6,6 @@ dune_python_install_package(
   CMAKE_METADATA_FILE dune/vtk/metadata.cmake        # metadata file to generate
   CMAKE_METADATA_FLAGS DUNE_OPTS_FILE
 )
-if ("${CMAKE_VERSION}" VERSION_GREATER 3.14)
+if(POLICY CMP0087)
   cmake_policy(SET CMP0087 NEW)
-endif ("${CMAKE_VERSION}" VERSION_GREATER 3.14)
+endif()