Leave the C11 keywords alone when we have a recent version of GCC.

As GCC also gained support for the C11 keywords over time, we can patch
up <sys/cdefs.h> to not define these anymore. This has the advantage
that error messages for static assertions are printed natively and that
_Alignas() will work with even a type outside of C11 mode.

All C11 keywords are supported with GCC 4.7 and higher, with the
exception of _Thread_local and _Generic. These are only supported as of
GCC 4.9.