meson: Make auto the default of the ssl option
The 'ssl' option is of type 'combo', but we add a choice 'auto' that
simulates the behavior of a feature option. This way, openssl is used
automatically by default if present, but we retain the ability to
potentially select another ssl library.
Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/
ad65ffd1-a9a7-fda1-59c6-
f7dc763c3051%40enterprisedb.com