build-sys: gettexts 0.18 -> 0.18.2 due to MKDIR_P

The autopoint is designed to not use the latest installed bug fix
release, if you specify

	AM_GNU_GETTEXT_VERSION([0.18])

in your configure.ac then it really uses 0.18, rather than also
installed 0.18.2 or 0.18.3. It means that bug fix update has no any
effect. Oh...

The AM_PROG_MKDIR_P is deprecated for years, unfortunately still used
in 0.18 gettext m4 stuff. This problem should be fixed in 0.18.2 where
is the correct AC_PROG_MKDIR_P macro.

If you do not have gettext 0.18.2 (or 0.18.3) then update.

Signed-off-by: Karel Zak <kzak@redhat.com>