C++11 is only recommended, not required

If we don't have a C++11 compiler, we won't be able to build DUNE, but
opm-core is still usable with for instance UMFPACK. However, if it *is*
available, it should be used to improve the ABI compatibility with other
parts of the suite.

CMakeLists.txt

@@ -29,7 +29,7 @@ include (OpmFind)
 # dependencies
 list (APPEND opm-core_DEPS
 	# compile with C++0x/11 support if available
-	"CXX11Features REQUIRED"
+	"CXX11Features"
 	# matrix library
 	"BLAS REQUIRED"
 	"LAPACK REQUIRED"