Add a warning that some functions are 2.8.12

cmake/modules/DuneEnableAllPackages.cmake

@@ -240,6 +240,15 @@ endfunction(dune_enable_all_packages)
 
 
 function(dune_target_enable_all_packages)
+  if (CMAKE_VERSION VERSION_LESS 2.8.12)
+    message(FATAL_ERROR "dune_target_enable_all_packages() needs CMake 2.8.12+")
+  elseif(CMAKE_MINIMUM_REQUIRED_VERSION VERSION_LESS 2.8.12)
+    message(WARNING
+"You are using dune_target_enable_all_packages().
+This requires at least CMake 2.9.12, but your Dune module only requires ${CMAKE_MINIMUM_REQUIRED_VERSION}.
+Update the cmake_minimum_required() call in your main CMakeLists.txt file to get rid of this warning.")
+  endif()
+
   foreach(_target ${ARGN})
 
     get_property(all_incs GLOBAL PROPERTY ALL_PKG_INCS)