Add cmake tests on polymorphic allocators

cmake/modules/CheckCXXFeatures.cmake

@@ -96,3 +96,11 @@ dune_check_cxx_source_compiles("
   #include <functional>
   int main() { std::identity{}; }
 " DUNE_HAVE_CXX_STD_IDENTITY)
+
+# Check for polymorphic allocators
+dune_check_cxx_source_compiles("
+#include <memory_resource>
+int main() {
+  auto mr = std::pmr::monotonic_buffer_resource{};
+  std::pmr::polymorphic_allocator<std::byte> alloc{&mr};
+}" DUNE_HAVE_CXX_STD_POLYMORPHIC_ALLOCATOR)

config.h.cmake

@@ -30,6 +30,9 @@
 /* does the standard library provide identity ? */
 #cmakedefine DUNE_HAVE_CXX_STD_IDENTITY 1
 
+/* does the standard library provide polymorphic allocators ? */
+#cmakedefine DUNE_HAVE_CXX_STD_POLYMORPHIC_ALLOCATOR 1
+
 /* Define if you have a BLAS library. */
 #cmakedefine HAVE_BLAS 1