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.

Signed-off-by: Daiki Ueno <dueno@src.gnome.org>