Build: Use AM_GNU_GETTEXT_REQUIRE_VERSION and require 0.19.6.

This bumps the version requirement from 0.19 (from 2014) to
0.19.6 (2015).

Using only the old AM_GNU_GETTEXT_VERSION results in old
gettext infrastructure being placed in the package. By using
both macros we get the latest gettext files while the other
programs in the Autotools family can still see the old macro.