This way it also compiles with older versions that don't define BOOST_FILESYSTEM_VERSION (ie, 1.42).
Stolen from https://bitbucket.org/squire_linden/viewer-dev-chop-662/changeset/6f971899f351
This introduces some API changes, like the new LLDirIterator, that causes changes elsewhere. It also include Log Linden's cache changes that aren't in viewer-development yet that increase the maximum cache size to ~10 GB.