Get bundled openssl to be used on OS X too, at least on OS X <10.7

Also note that OpenSSL is incompatible with -fvisibility=hidden on OS X

Change-Id: I1fc3cf4d515ef210f996cb04ba9c1b3216109137
Reviewed-on: https://gerrit.libreoffice.org/7233
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>