lib/crypto: Detect CommonCrypto and use it if available

lib/crypto: Detect CommonCrypto and use it if available

CommonCrypto/CommonDigest is available on Mac and there is function in
the libc for MD5 calculation. MD5Final is a C define of CC_MD5_Final.
Under some circumstance we have the symbol defined twice in samba
binaries on Snow Leopard at least.

By detecting CommonCrypto/CommonDigest we end up always using the system
version if available.