configure: Add GnuTLS to configure overview

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
configure: Disable python3 builds with clang

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>