gettext.m4: Recognize a statically built libintl on macOS and AIX.
Reported by David Edelsohn <dje.gcc@gmail.com> in
<https://gcc.gnu.org/pipermail/gcc-patches/2023-November/636558.html>.
* gettext-runtime/m4/gettext.m4 (AM_GNU_GETTEXT): In the
"checking for GNU gettext in libintl..." test, add also the special
system library dependencies needed on macOS and AIX.
