Spotting compilation-time bottlenecks in order to

Question

I have this big C++ boostified project that takes ages to build so i'm trying to set up compilation firewalls. Now I could sprinkle pimpls or pure interfaces following my intuition but that doesn't seem very efficient... usually if i wanted to optimize a piece of code, I would run it through a profiler to see the bottlenecks which leads me to the following question: how do I see where are the bottlenecks in my compilation time?

All answers including trying alternate compilers are welcomed since code is cross-platform (crosses fingers!)

Thanks,