Updated.  I didn't go for actually removing 4.8 from the supported compilers, as it seemed like supporting the alternate BSD names (like "g++48") would get quite hacky.  It would be easy enough to put a warning in either here or in compiler.rb, though.

I'm not sure what that warning should be.  The issue with disabling werror, as I see it, is that there is a higher possibility that a problem with the build environment (and libv8's assumptions about the system libraries in particular) could go unnoticed, and as a result a buggy binary be built.