Add explanation for config.h include

dune/common/std/functional.hh

@@ -7,7 +7,7 @@
 
 #include <functional>
 
-#include <dune-common-config.hh>
+#include <dune-common-config.hh> // DUNE_HAVE_CXX_STD_IDENTITY
 
 namespace Dune
 {

dune/common/std/type_traits.hh

@@ -6,7 +6,7 @@
 #define DUNE_COMMON_STD_TYPE_TRAITS_HH
 
 #include <type_traits>
-#include <dune-common-config.hh>
+#include <dune-common-config.hh> // DUNE_HAVE_CXX_EXPERIMENTAL_IS_DETECTED
 #include <dune/common/typetraits.hh>
 #include <dune/common/typeutilities.hh>
 

dune/common/stdstreams.hh

@@ -15,7 +15,7 @@
 #ifndef DUNE_COMMON_STDSTREAMS_HH
 #define DUNE_COMMON_STDSTREAMS_HH
 
-#include <dune-common-config.hh>
+#include <dune-common-config.hh> // DUNE_MINIMAL_DEBUG_LEVEL
 #include "debugstream.hh"
 
 namespace Dune {

dune/common/version.hh

@@ -5,7 +5,7 @@
 #ifndef DUNE_VERSION_HH
 #define DUNE_VERSION_HH
 
-#include <dune-common-config.hh>
+#include <dune-common-config.hh> // DUNE_[MODULE]_VERSION_[TYPE]
 
 /** \file
  * \brief Various macros to work with %Dune module version numbers