The CMake build system has not been maintained for years and does not
work correctly anymore. Rather than keeping it here as a "trap" for new
users, I'll remove it for now. FWIW Mitsuba 2 will have a much nicer
CMake build system that works consistently on all platforms.

Fix boost include error occurring in Ubuntu 16.04.

sunmodel: accept animated transformations (even if animations aren't directly supported within SunModel)

Fixed an incorrect radiance conversion factor in 'coating' and 'roughcoating' reported by Attila Afra.

The models now produce the right falloff compared to Monte Carlo
reference renderings with explicitly instantiated layers

Implementation of the paper 'Accurate computation of single scattering in participating media with refractive boundaries' contributed by Nicolas Holzschuch