It is only "enforced" for the paddock and then laxly. If you avoid shorts and obviously scruffy clothes then my exprience suggests that you'll have no problem. However, imho, it is nice to see some of the guys and gals who make an effort and dress up. Mr T and Paul usually wear collared shirts and light jackets over but I've seen some really scruffy herberts wandering about unchallenged.
The code is, in effect, smart casual.
EDIT : Here you go:
http://www.lemansclassic.com/dress-code