Fix 'ar' invocation when cross-compiling and in 64-bit mode on AIX.

Fix 'ar' invocation when cross-compiling and in 64-bit mode on AIX.

Reported by Benedikt Morbach <bmorbach@redhat.com>
at <https://savannah.gnu.org/bugs/?43037>.

* gettext-runtime/m4/intl.m4 (AM_INTL_SUBDIR): Require gl_PROG_AR_RANLIB if
available. Otherwise, use AC_PROG_RANLIB and set AR and ARFLAGS.
* gettext-runtime/intl/Makefile.in (AR): Use the value determined through
autoconf.
(ARFLAGS): New variable.
(libintl.a, libgnuintl.a): Use it.
* gettext-runtime/configure.ac: Invoke gl_PROG_AR_RANLIB.
* gettext-runtime/libasprintf/configure.ac: Likewise.
* gettext-tools/configure.ac: Likewise.