Allow the Makefile to determine the state of PERL_THREADED.

While I'm here, you cannot rely on the value of a define to be
something specific, it is either defined or it is not.

No PORTREVISION bump because WITH_THREADS is not the default, and for
those attempting to use this option and/or related options the port has
not built since the last update.

Approved by: lack of response from maintainer, problem affecting users