Add include directories to library target

This properly sets up the dune-alugrid 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::ALUGrid (or dunealugrid) 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. (this was done already for all tests in dune-alugrid)

CMakeLists.txt

@@ -45,12 +45,16 @@ list(APPEND CMAKE_MODULE_PATH ${dune-common_MODULE_PATH}
 #include the dune macros
 include(DuneMacros)
 
+# deactivate global include-directories
+dune_policy(SET DP_DEFAULT_INCLUDE_DIRS NEW)
+
 # start a dune project with information from dune.module
 dune_project()
 
 # Define the dune-alugrid library
 dune_add_library(dunealugrid EXPORT_NAME ALUGrid)
 target_link_libraries(dunealugrid PUBLIC Dune::Grid)
+dune_default_include_directories(dunealugrid PUBLIC)
 
 add_subdirectory(cmake/modules)