Add aligned allocator support to meson

Add aligned allocator support to meson

* A bunch of new variables for config.h.meson
* A check for aligned allocation being necessary at all
  (graphene must use GCC vector instructions or SSE2)
* A check for C malloc() being aligned at 16-byte boundaries
* A check for a few special aligned allocator functions being
  present and not being built-ins (posix_memalign is a builtin
  in GCC, even on platforms where there is no posix_memalign
  system function)
* Added -mstackrealign flag on Windows, since otherwise
  stack variables may become unaligned when the stack briefly
  passes through OS code (such as in various callbacks and
  handlers)