Add include directories to library target

This properly sets up the dune-foamgrid main target to follow modern CMake practices where the target holds all the information to propagate transitively to other targets. The MR also suppresses the warning that a Dune policy is not set, by setting it to NEW. The consequence is that there is no global include_directories(...) call in dune_project anymore and one needs to link all targets that need Dune include directories either to the library target Dune::FoamGrid (or dunefoamgrid) or set the include directories manually, e.g., using the utility function dune_default_include_directories(<target> [PUBLIC|PRIVATE|INTERFACE]). This is especially important for tests. That is why tests should be linked against the library target.

CMakeLists.txt

@@ -15,8 +15,10 @@ list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake/modules"
 #include the dune macros
 include(DuneMacros)
 
-# deactivate global include-directories
-dune_policy(SET DP_DEFAULT_INCLUDE_DIRS NEW)
+if(dune-common_VERSION VERSION_GREATER_EQUAL 2.11)
+  # deactivate global include-directories
+  dune_policy(SET DP_DEFAULT_INCLUDE_DIRS NEW)
+endif()
 
 # start a dune project with information from dune.module
 dune_project()
@@ -24,8 +26,10 @@ dune_add_library(dunefoamgrid INTERFACE
   EXPORT_NAME FoamGrid
   LINK_LIBRARIES Dune::Grid)
 
-# set include directories for dunefoamgrid library
-dune_default_include_directories(dunefoamgrid INTERFACE)
+if(dune-common_VERSION VERSION_GREATER_EQUAL 2.11)
+  # set include directories for dunefoamgrid library
+  dune_default_include_directories(dunefoamgrid INTERFACE)
+endif()
 
 add_subdirectory(dune)
 add_subdirectory(doc)