Autotools support for SoftEther

This is my first draft of autotools for SoftEther.  The motivation for this enhancement is:

* Easier native packaging.
* Better FHS conformance.
* Portability for embedded systems, especially those that don't use glibc.
* Dyamic linking.  SoftEther can easily fit into a 2 megabyte package.

If this kind of work is acceptable for upstream, then I will finish it and update this pull request.