configure.ac: bump required gettext version to 0.19
0.19 should be common
see
@@ -18,7 +18,7 @@ AC_PROG_CC
AM_PROG_CC_C_O
AM_GNU_GETTEXT([external])
-AM_GNU_GETTEXT_VERSION([0.18])
+AM_GNU_GETTEXT_VERSION([0.19])
# Fix `undefined reference to `libintl_gettext'` on docker:
AC_CHECK_LIB([intl], [libintl_dgettext])
