Fix SIZET_SAT_ADD for the case of size_t is signed (SunOS 4.X)

Fix SIZET_SAT_ADD for the case of size_t is signed (SunOS 4.X)

Negative size_t value is impossible in a correct C implementation,
but quite possible under SunOS 4.X.

* src/atomic_ops_malloc.c: Include limits.h (unless SIZE_MAX already
defined).
* src/atomic_ops_malloc.c (AO_SIZE_MAX): New macro.
* src/atomic_ops_malloc.c (SIZET_SAT_ADD): Use AO_SIZE_MAX instead of
~(size_t)0.