Mac OS X Lion support

[Douglas Creager]

> It looks like you're setting JEMALLOC_ZONE_VERSION as a compile-time
> constant?  Unless you never intend to run the program on a system
> other than the one it was compiled on, I don't think this will work.

Yep, but that's what the standalone jemalloc was already doing previously.  The current dev branch (and the 2.2.3 release) do the configure-time check, and build a libjemalloc.dylib that only works on the current system.  I assumed that it's a configure-time check for a reason, so I kept it that way.

–doug