We don't do a CheckLib for SSL. We just add ssl and crypto to LIBS. The result is that if the libssl and libcrypto libraries are not present, later TryLink or CheckLib calls fail. This can lead to confusing error messages complaining that, for instance, boost cannot be found, even though the real problem is that the SSL libraries are not available.
The fix is to add a proper CheckLib for the SSL libraries and error out if they are not found but the user requested SSL support with --ssl.