* guix/build/download.scm: Autoload (gnutls).
(tls-wrap): New procedure.
(open-connection-for-uri): Add support for `https'. Wrap the socket
with `tls-wrap' in that case.
(url-fetch): Add `https'.
* guix/download.scm (gnutls-derivation): New procedure.
(url-fetch)[need-gnutls?]: New variable.
Call `gnutls-derivation' when NEED-GNUTLS? is true, and add its output
to the `GUILE_LOAD_PATH' env. var. in that case.
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