facebook/hhvm — pull request #2919 by denji (2014-06-12 01:18:38+00:00)
Title: Detect gnu++11/gnu++1y features

Detect Gnu/C++14 define `ifdef (_CPP14)`

- https://gcc.gnu.org/projects/cxx1y.html
- http://clang.llvm.org/cxx_status.html
- [Working Draft, Standard - C++1y/C++14](http://isocpp.org/files/papers/N3797.pdf)
- https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2011
- https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2014
- https://gcc.gnu.org/onlinedocs/libstdc++/manual/using.html#manual.intro.using.flags