The functions `is_valid` and `is_simple` and their associated
headers are only present in versions of Boost >= 1.56, and will
cause compilation errors with previous versions of Boost. This
masks them with preprocessor macros to allow the compilation to
complete, at the cost of some (currently not widely used)
functionality.
- the original motivation for this was to avoid needing to
mutate a copy later on (for operations needing correct winding order)
- but mutating a copy is looking feasible, so removing this now.
