Revert "Demote consequence of not having C++11 support"

This reverts commit d6c2e52d63. The C++11
find module now checks for the necessary level.

cmake/Modules/Finddune-common.cmake

@@ -16,7 +16,7 @@ find_opm_package (
   "dune-common"
 
   # dependencies
-  "CXX11Features;
+  "CXX11Features REQUIRED;
   BLAS REQUIRED;
   LAPACK REQUIRED;
   MPI

cmake/Modules/Finddune-geometry.cmake

@@ -19,7 +19,7 @@ find_opm_package (
   # TODO: we should probe for all the HAVE_* values listed below;
   # however, we don't actually use them in our implementation, so
   # we just include them to forward here in case anyone else does
-  "CXX11Features;
+  "CXX11Features REQUIRED;
   dune-common REQUIRED
   "
   # header to search for

cmake/Modules/Finddune-grid.cmake

@@ -19,7 +19,7 @@ find_opm_package (
   # TODO: we should probe for all the HAVE_* values listed below;
   # however, we don't actually use them in our implementation, so
   # we just include them to forward here in case anyone else does
-  "CXX11Features;
+  "CXX11Features REQUIRED;
   dune-common REQUIRED;
   dune-geometry REQUIRED
   "