#93 buildable without gcrypt

[Andreas Metzler]

Hello,

aria 1.5.1 requires both gcrypt and gnutls. Because current versions of gnutls do not use gcrypt this adds an additional external dependency. Afaict aria only uses a single gcrypt function (gcry_md_hash_buffer). If it used the corresponding funktion in gnutls crypto API (gnutls_hash_fast) instead the unnecessary external dependency could be dropped.

cu Andreas