Don't misdetect stpcpy on windows platforms on clang

See https://github.com/mesonbuild/meson/issues/3672 and https://github.com/mesonbuild/meson/issues/5628 for explanations of cases where meson misdetects functions due to clang builtins (that always are available, regardless of whether the platform actually provides them).

This is admittedly a bit ugly and I can see if you'd prefer not to merge it, but downstream users at least need a patch like this until the misdetection is fixed in meson (and the older issue has been open for soon 2 years now) - thus proposing it here for discussion.