maint: port to bleeding-edge Automake by assuming Gettext 0.18.2

Reported by Stefano Lattarini in
<http://lists.gnu.org/archive/html/bug-tar/2013-04/msg00009.html>.
* configure.ac (AM_GNU_GETTEXT_VERSION): Increase from 0.16 to 0.18.2.
README-alpha says that we do not make any efforts to accommodate
older versions of Gettext.

configure.ac

@@ -345,7 +345,7 @@ AC_CHECK_TYPE(iconv_t,:,
 
 # Gettext.
 AM_GNU_GETTEXT([external], [need-formatstring-macros])
-AM_GNU_GETTEXT_VERSION([0.16])
+AM_GNU_GETTEXT_VERSION([0.18.2])
 
 # Initialize the test suite.
 AC_CONFIG_TESTDIR(tests)