Update to a newer gettext to fix clang issues

See also: https://bugs.gentoo.org/908414
See also: https://github.com/gentoo/gentoo/pull/32491
--- a/configure.ac
+++ b/configure.ac
@@ -17,8 +17,8 @@ AC_SYS_LARGEFILE
 dnl AC_PROG_LIBTOOL
 AC_CONFIG_MACRO_DIR([m4])
 AC_C_BIGENDIAN
-AM_GNU_GETTEXT
-AM_GNU_GETTEXT_VERSION([0.18])
+AM_GNU_GETTEXT([external])
+AM_GNU_GETTEXT_REQUIRE_VERSION([0.21])
 PKG_PROG_PKG_CONFIG
 AM_PROG_CC_C_O
 AM_ICONV