build: Try to use latest gettext available

Using AM_GNU_GETTEXT_REQUIRE_VERSION specifies the minimal required
gettext version, but tries to use tha latest supported version
available on the system, so we can take advantage of any fixes and
improvements.

We cannot remove AM_GNU_GETTEXT_VERSION() for backwards compatibility
reasons, otherwise older autopoint and autoreconf versions would not
be able to know what is the minimal required gettext versions.

configure.ac

@@ -14,6 +14,14 @@ AC_USE_SYSTEM_EXTENSIONS
 AM_INIT_AUTOMAKE([1.11 -Wall foreign nostdinc no-dist-gzip dist-xz])
 AM_SILENT_RULES([yes])
 
+# Do not unconditionally use the following macro, as Debian jessie does not
+# have a recent enough gettext.
+m4_ifdef([AM_GNU_GETTEXT_REQUIRE_VERSION],
+         [AM_GNU_GETTEXT_REQUIRE_VERSION([0.19])])
+# We cannot remove the following macro due to backwards compatibility reasons.
+# In the future when the gettext version is bumped, we have to set the above
+# macro unconditionally to the minimal version, and set the below to 0.19.6,
+# the first version introducing the AM_GNU_GETTEXT_REQUIRE_VERSION macro.
 AM_GNU_GETTEXT_VERSION([0.19])
 AM_GNU_GETTEXT([external])
 DPKG_DIST_CHECK([test "$GMSGFMT" = ":" && test "$USE_NLS" = "yes"],

debian/changelog

@@ -29,6 +29,8 @@ dpkg (1.18.4) UNRELEASED; urgency=low
     - Descend into scripts directory when cleaning up code coverage files.
     - Add new configure option --disable-devel-docs to select the kind of docs
       to generate, default for now is development documentation.
+    - Try to use AM_GNU_GETTEXT_REQUIRE_VERSION to benefit from the latest
+      installed gettext version, while guaranteeing a minimal required version.
   * Documentation:
     - Move description for “target architecture” from the dpkg-architecture(1)
       ‘-A’ option to the TERMS section. Closes: #799046