add config.opts file to metadata and enable CMP0087 NEW cmake policy

comment out CMP0087 NEW cmake policy because it fails on some CI runners

correctly set CMP0087 cmake policy to NEW for cmake >= 3.14

python/CMakeLists.txt

@@ -4,4 +4,8 @@ dune_python_install_package(
   PATH "."
   CMAKE_METADATA_FILE dune/istl/metadata.cmake
   DEPENDS _istl
+  CMAKE_METADATA_FLAGS DUNE_OPTS_FILE
 )
+if ("${CMAKE_VERSION}" VERSION_GREATER 3.14)
+  cmake_policy(SET CMP0087 NEW)
+endif ("${CMAKE_VERSION}" VERSION_GREATER 3.14)