The definitions in _stdint.h has some complications around visibility
that _limits.h does not have. Switch to __SIZE_T_MAX to avoid those.
This fixes the devel/gperf, devel/glib20 and math/mpfr builds with
_FORTIFY_SOURCE enabled to unlock a large fraction of the ports tree to
build.
Reported by: Shawn Webb (HardenedBSD)
Sponsored by: Klara, Inc.
Sponsored by: Stormshield