gettext: avoid obsolete macro AM_PROG_MKDIR_P

gettext: avoid obsolete macro AM_PROG_MKDIR_P

It is obsolete and is planned to be removed from Automake 1.14; see
<http://lists.gnu.org/archive/html/automake/2012-12/msg00029.html>.
* build-aux/po/Makefile.in.in (install-data, install-data-yes)
(installdirs-data, installdirs-data-yes):
Use $(MKDIR_P), not $(mkdir_p).
* m4/intl.m4 (AM_INTL_SUBDIR):
* m4/po.m4 (AM_PO_SUBDIRS):
Require AC_PROG_MKDIR_P, not AM_PROG_MKDIR_P.