Permit building with old autoconf.

There is a compatibility macro added to varnish.m4 in 4.0.3,
so we can build VMODs on EL6 now.

configure.ac

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