Merge pull request #4025 from RosettaCommons/JackMaguire/ClangCxx14
Correcting extras=cxx... flags for clang
#3992 added options to compile with std=c++1y and later versions of c++. To do this, I needed to remove std=c++1x from those builds but failed to notice that clang uses std=c++11 instead. This PR simply corrects the flag being removed.
Thanks for the review Sergey!