Compatibility improvements for different boost filesystem versions

Previously we moved to v3 of the boost filesystem library, because v2 is no longer supported in versions >= 1.50. But we still need to build with boost 1.43 to support Arnold on OS X, which links to that version and doesn't hide the symbols, and filesystem v3 wasn't introduced until boost 1.44. This fix reinstates support for 1.43, and also removes all use of other deprecated boost::filesystem functionality, now building cleanly with BOOST_FILESYSTEM_NO_DEPRECATED.

I've only tested this on OS X with boost 1.43.0, so it needs testing with the fuller set of configurations at Image Engine.