Revert "Disable fibers on >= 10.7. Fixes #815"

This reverts commit e70c097e4f58130efba7b5f845fba9e16ae3c4eb.

This should be enabled again, so for example Process.cpu_times works
properly for current thread timings. The original reason for this change
is a nonexistant reason now.
Fixed compilation on Mavericks broken by revert in b83a17b84.
Merge remote-tracking branch 'origin/master' into compiler