meson: fix OpenSSL fallback when not explicitly required

meson: fix OpenSSL fallback when not explicitly required

When OpenSSL isn't provided by the system we know to fall back to the
subproject wrapper. This is especially helpful on Windows systems, where
you typically don't have OpenSSL available, in order to reduce the
number of required dependencies.

The fallback is broken though when the OpenSSL backend is set to 'auto'
as we end up calling `dependency('openssl', required: false)` in that
case, which implicitly disables falling back to the wrapper.

Fix the issue by re-allowing the fallback in case either OpenSSL is
required or in case the backend is set to 'auto'. While at it, fix
reporting of the backend in case the user asked us to pick no HTTPS
backend at all.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>