`>=glibc-2.38` implements strlcpy, strlcat, etc so check for those functions
with Meson and don't provide conflicting prototypes.
Technically, it doesn't need _GNU_SOURCE, but it's easier because it's not
clear right now what glibc wants to guard it with. Note that these are in
POSIX next anyway.
Fixes: https://github.com/OpenRC/openrc/issues/643
Signed-off-by: Sam James <sam@gentoo.org>
c1cd3c9830