Switch the curl bindings from optional to required

Currently, we support curl being optional via some sporting exercises:
ifdefs in the code, data types that represent 'Curl is disabled'
state, etc. However, with the future work on RPC, we would have to
even make the dependencies list conditional on it, etc. This is too
much work, when the curl library is included even in stable
distributions.

This patch changes curl from option to required, the same as the other
base libraries.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>