Disable lang/perl5 threads by default

[Jonathan Perkin <jperkin%joyent.com@localhost>]

For the last couple of releases, Perl has officially discouraged the
use of ithreads:

  
https://metacpan.org/changes/distribution/perl#Interpreter-based-threads-are-now-discouraged

Our options.mk enables threads by default if they are builtin, which
is probably the vast majority of platforms by now.

I propose we remove the PKG_SUGGESTED_OPTIONS setting, and leave it to
the user to enable them if required, so that we match the upstream
recommendation.

Objections?

-- 
Jonathan Perkin  -  Joyent, Inc.  -  www.joyent.com