Bump autoconf minimum version to 2.64

See: https://github.com/coreutils/gnulib/blob/master/NEWS#L6

configure.ac

@@ -1,5 +1,5 @@
 dnl Process this file with autoconf to produce a configure script.
-AC_PREREQ([2.63])
+AC_PREREQ([2.64])
 AC_INIT([lftp],m4_esyscmd([build-aux/git-version-gen .tarball-version]),[lftp-bugs@lftp.yar.ru])
 AC_CONFIG_AUX_DIR([build-aux])
 AC_CONFIG_MACRO_DIR([m4])