Support GnuTLS as an alternative crypto backend

Daiki Ueno requested to merge wip/dueno/crypto-backend into master

This turns the -Dgcrypt build time option into a more generic -Dcrypto option, which enables user to choose which cryptographic library to link with. It currently supports libgcrypt (libgcrypt) and GnuTLS (gnutls); for the latter, GnuTLS 3.8.2 is the minimum required version.

Edited by Daiki Ueno

Merge request reports