Respect the configure option --localedir.
Reported by Markus Gothe at https://savannah.gnu.org/bugs/index.php?49862 .
The AC_SUBSTed variable @localedir@ is supported since Autoconf 2.60.
* gettext-runtime/intl/Makefile.in (localedir): Set to @localedir@. Suggested
by Markus Gothe <nietzsche@lysator.liu.se>.
* gettext-runtime/m4/intldir.m4: Require Autoconf 2.60 or newer.
* gettext-tools/doc/gettext.texi (src/Makefile): Assume Autoconf 2.60 or newer.
