Only display an error message if C++11 support is noted as REQUIRED
Revert "Demote consequence of not having C++11 support"

This reverts commit d6c2e52d638af3b4783dfa5efa01fb76675b1499. The C++11
find module now checks for the necessary level.
OPM Core uses C++11 features as of commit 562bcf2
Merge pull request #295 from rolk/295_cxxreq

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