libxmlsec1 1.2.20

Version bump. I’ve also moved from GnuTLS to OpenSSL, because even when
configured explicitly with GnuTLS it was still pulling in the system
OpenSSL and using that in part anyway, and having both crypto libraries
as dependencies just seemed redundant when libxmlsec1 allows you to use
either. If anyone strongly objects I guess I’m open to having both
libraries as dependencies.
libxmlsec1: update 1.2.20 bottle.