[PATCH C++, PING] - SD-6 Implementation Part N/4.

[Ed Smith-Rowland]

I would like review of my SD-6 implementation.

Part 1 - __has_include__ built-in in libcpp.
https://gcc.gnu.org/ml/gcc-patches/2014-09/msg00083.html

Part 2 - __has_include macro and C++ language feature macros.
https://gcc.gnu.org/ml/gcc-patches/2014-09/msg00084.html

Part 3 - libstdc++ library macros was already approved by Jonathan. 
Thank you.
https://gcc.gnu.org/ml/gcc-patches/2014-09/msg00085.html
https://gcc.gnu.org/ml/gcc-patches/2014-09/msg00110.html

Part 4 - C++ front end tests.
https://gcc.gnu.org/ml/gcc-patches/2014-09/msg00553.html

These build and test cleanly on x86_64-linux.

Part 5 - __has_cpp_attribute and supporting machinery is proving more 
interesting than I thought.
But the rest should be put in.

Thank you,
Ed Smith-Rowland