Only fail if C++11 support is noted as REQUIRED

Note that such a requirement will propagate; if A requires B and B requires C++11 support, then A (still) cannot build.

This (currently) slack the requirement on opm-core, but still works as intended for the rest. (And if we want opm-core to fail if there is not C++ support, then we should really mark it as REQUIRED in the prereq list).