meson: fix openssl detection issues in 6a30027

meson: fix openssl detection issues in 6a30027

When not detecting openssl via pkg-config, we'd error out if the headers
weren't found, even if -Dssl=auto. When detecting via pkg-config, but the
headers could not be found, we'd error out because the ssl_int variable would
not exist.

Reported-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/20230313180432.GA246741@nathanxps13