clang++

The new mac os compiler seems to be a huge improvement over gcc. In addition to much nicer error messages and faster compilation, I can build IMP from scratch in 20 minutes on my 2 year old laptop, keeping all 4 cores busy. And the laptop remains completely usable for other purposes the whole time (as compared to building IMP with gcc which locks up my laptop and linux box for extended periods and tends to spend inordinate amounts of time waiting for I/O).