Only use the libcrypto part of OpenSSL #58

We shouldn't be needing libssl at all; all functions we use are in libcrypto.

@theuni Care to review?

@sipa: Hmm, I thought I tested this when doing the autotools stuff and openssl was necessary for some reason. Looking.

openssl = libssl + libcrypto. We only use things from libcrypto.

@sipa Looks like I was thrown off at the time by the fact that libcrypto installs headers under openssl/, so I assumed that they may be dependent on some openssl headers (global build config headers, for ex) as a result.

I've tried building and installing libcrypto without libssl to no avail, and distros don't attempt to split them up either. So it seems there's no risk of missing any parent headers. ACK on the change.

Worth renaming the config switch while it's still early to --with-bignum=libcrypto ?

@theuni I think libcrypto is less clear than openssl, and as they're inseparable as part of that system, there is little difference anyway.

@theuni It's the other way around btw: libssl (high level ssl implementation) depends on libcrypto (low level crypto routines).

@sipa sounds good. As for the split, I realize that libssl depends on libcrypto. However, that's not to say that libcrypto doesn't end up sharing some global config headers from the parent project.

For ex, bitcoin's crypto/* files depend on bitcoin-config.h to determine what features it can use. Similarly, libcrypto relies on openssl/opensslconf.h (openssl/crypto.h -> openssl/e_os2.h -> openssl/opensslconf.h). But they're inseparable anyway, so that's no concern to us.