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.
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