digest: add OpenSSL implementation

digest: add OpenSSL implementation

After studying the available Max OS X documentation it has become clear
that the APIs offered by the Security framework are too incomplete to
cover the SIPE requirements.

Luckily there is a fallback option: OpenSSL. Although declared
deprecated by Apple it at least is available out-of-the-box on Mac OS X
and thus doesn't require any special build setup. At the same time the
new code can also be written and tested on Linux.

Start off by implementing the simple stuff:
- digests
- temporary configure hack to detect OpenSSL headers and libraries.