obfsproxy: support autoconf 2.59
The attached patch (against git commit bf9c087ec53e60bc203fa02374a2b7707e61a14f) allows obfsproxy to successfully be autogenned using autoconf 2.59, the latest version available on CentOS 5.x systems. The commenting of the sed check probably isn't ideal but I was working quickly to try to get it running on a system - it should be relatively simple to add a custom sed check if I understand correctly, I just can't poke at it further right now.