Require recent autoconf.

On EL6 systems the packaged 2.63 version is missing AS_VAR_COPY.
This seems to have been added in 2.63b, released sometime in 2008.

With this commit we cut off any vmod development being done on
EL6 and earlier. (building and running should work)

configure.ac

-AC_PREREQ(2.59)
+AC_PREREQ(2.64)
 AC_COPYRIGHT([Copyright (c) 2011-2014 Varnish Software AS])
 AC_INIT([libvmod-example], [trunk])
 AC_CONFIG_MACRO_DIR([m4])