[pdal] Boost versioning

[Michael P. Gerlek]

Yesterday Mr. Rosen found yet another problem with Boost compatibility (1.47.x against VS2010)

This is a topic for the Sprint next week, but maybe we can jump start it here...

- We need to support whatever version the major linux distros are using
- We want to support the (relatively) latest-and-greatest release (so we get nifty stuff we can gratuitously splatter all over the codebase :-)

Can we then cut this problem down to just supporting two versions?  Or are there other concerns?  (like, which boost do the Mac fanboys use?)

-mpg