[xserver] os: Add libsha1 as a choice of SHA1 implementation

[Mark Kettenis]

> From: Mikhail Gusarov <dottedmag at dottedmag.net>
> Date: Thu, 29 Oct 2009 00:40:27 +0600
> 
> There are small systems which don't need OpenSSL or gcrypt.
> Add libsha1 (http://github.com/dottedmag/libsha1) as an alternative
> small SHA1 implementation.

This makes me seriosy wonder whether instead of adding all this glue
and associated autoconf logic we should just include a suitably
licenced sha1 implementation and be done with it.