Also don't check for a default_trust_store_file in configure when building on
macOS (unless explicitly asked to with --with-default-trust-store-file=xxx),
because otherwise it finds /etc/ssl/cert.pem: This file is new (since
10.12.2?), which means libraries built on the newest OS version wouldn't work
the same way on an older versions (and vice versa).  "/etc/ssl/cert.pem" also
doesn't seem to reflect additions and deletions from the user's or system's
trusted roots keychain (in my limited testing).

Signed-off-by: David Caldwell <david@porkrind.org>