Explicitly support building against libssh2 built with libgcrypt

Signed-off-by: James McCoy <vega.james@gmail.com>
Setup threading callbacks for gcrypt

Signed-off-by: James McCoy <vega.james@gmail.com>
Setup threading callbacks for OpenSSL

Signed-off-by: James McCoy <vega.james@gmail.com>
Merge pull request #10 from jamessan/pu/threadsafety

Threading initialization for crypto libraries