gnutls: remove 3.5.10
Nix packages are expected to honor NIX_SSL_CERT_FILE and this removes the
dependency on the framework while bootstrapping the stdenv.
(+ nitpick changes from vcunat)
The patch is based on https://gitlab.com/gnutls/gnutls/commit/c0eb46d3463cd21b3f822ac377ff37f067f66b8d