Detect gnu++11/gnu++1y features

Detect Gnu/C++14 define `ifdef (_CPP14)`

- [Current Status : ISO Standard C++](https://isocpp.org/std/status)
- [C++1y/C++14 Support in GCC](https://gcc.gnu.org/projects/cxx1y.html)
- [C++14, C++11 and C++98 Support in Clang](http://clang.llvm.org/cxx_status.html)
- [Working Draft, Standard - C++1y/C++14](http://isocpp.org/files/papers/N3797.pdf)
- [Implementation Status C++ 2011](https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2011)
- [Implementation Status C++ 2014](https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2014)
- [ GNU C++ library is shaped by several GCC Command Options](https://gcc.gnu.org/onlinedocs/libstdc++/manual/using.html#manual.intro.using.flags)