BUG: Pass system search paths from system build to external builds

This enable the restriction of places searched for distributions
systems such as Homebrew or Anaconda Python to be restricted from
searching the OS system paths.

Change-Id: I697d10bfd345da8c1e3acec259e5685712abbff9
Merge topic 'SIMPLEITK-581_PassSearchPathsInSuperbuild' into next

0ce5473b BUG: Pass system search paths from system build to external builds