Update stddef.h

Fix for clang build for C++11 using gcc 4.9 standard library on Linux (https://github.com/cilkplus/clang/issues/15).
Update c11.c

Fix for clang build for C++11 using gcc 4.9 standard library on Linux (https://github.com/cilkplus/clang/issues/15).
Merge pull request #1 from cilkplus/cilkplus

Pull latest changes