Add a -fghci-sandbox flag so that we can en/disable the ghci sandbox

Add a -fghci-sandbox flag so that we can en/disable the ghci sandbox
It's on by default (which matches the previous behaviour).

Motivation:
GLUT on OS X needs to run on the main thread. If you
try to use it from another thread then you just get a
white rectangle rendered. For this, or anything else
with such restrictions, you can turn the GHCi sandbox off
and things will be run in the main thread.