[Bug c/69680] stdlib.h does not declare aligned_alloc

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69680

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jeff Hammond from comment #0)
> It seems that none of the GCC 5.3.0 headers declare this function...

Because GCC doesn't provide a C library, it uses the one from your OS.
Presumably it doesn't support C11, or at least not aligned_alloc.