AppleClang: Use modern C++14 standard flags for Apple Clang 6.1

The Apple Clang 6.1 compiler that comes with Xcode 6.3 is aware of the
modern -std=c++14 and -std=gnu++14 flags, so use them instead of the
"1y" flags.

Suggested-by:  <darkapostle@rule506.net>