Minimum supported version of Forte

Unless anyone has any major objections, we're going to start treating 
the Forte 6U2 release as the minimal supported version of the Forte 
compiler that we can use to compile Mozilla.   Forte 6U2 is what the 
nebiros tinderbox is using.  I don't plan on adding any checks to 
enforce this change unless it becomes a problem.  We're also going to 
bump the default optimization level for Forte builds to -xO4.

http://bugzilla.mozilla.org/show_bug.cgi?id=109860

- cls