Isn't it pronounced "Ligh-ster"?
I believe the spelling was the french spelling of the Native American word they used for the area, Michilimackinac (pronounced MiSHILLeeMAKinaw). It originally referred to the entire area and after Marquette founded a mission there the village became known as "Michilimackinac".
Here's a couple interesting links on the subject:
http://turtletrack.org/CO_FirstPerso...kbird_Ch03.htm
http://www.answers.com/topic/michilimackinac
http://www.answers.com/topic/mackinac-island