Adjust download-mirror URI to new mirror layout.

* The ports mirror at http://ports-mirror.haiku-os.org is now using
  per-recipe subdirs to handle bad download target filenames more
  gracefully.
Shutup policy warning for cross-builds.

* Only show a warning about Haiku-libraries not being found in 
  /boot/system/lib/ when we are actually building on Haiku.