build: raise gettext version requirement

build: raise gettext version requirement

Raise gettext requirement to the latest oldstable version 0.18.3.
This fixes the following automake warning:

configure.ac:581: warning: The 'AM_PROG_MKDIR_P' macro is deprecated, and its use is discouraged.
configure.ac:581: You should use the Autoconf-provided 'AC_PROG_MKDIR_P' macro instead,
configure.ac:581: and use '$(MKDIR_P)' instead of '$(mkdir_p)'in your Makefile.am files.

* configure.ac (AM_GNU_GETTEXT_VERSION): Raise from 0.15 to 0.18.3.
* po/Makevars: Update from gettext-0.18.3.