Allow building with gettext ≥ 0.20
The use of AM_GNU_GETTEXT_VERSION in configure.ac instructs autopoint to copy po/Makefile.in.in from the exact gettext version. It is fine if the version of gettext installed on the system has the same minor version number with the requested version, but it fails if you have a newer version of gettext because of the mismatch between autoconf macros and Makefile.in.in.
*** error: gettext infrastructure mismatch: using a Makefile.in.in from gettext version 0.19 but the autoconf macros are from gettext version 0.20
Instead of specifying the exact version with AM_GNU_GETTEXT_VERSION, we can use AM_GNU_GETTEXT_REQUIRE_VERSION to ask autopoint to simply use the gettext version installed on the system to prevent the mismatch.
This patch is based on geocode-glib@6ba0e38f.