Starting with dev-libs/boost-1.46 the default version of boost.filesystem is 3. Packages using version 2 need to explicitly say so or will fail to compile. If you package has this problem use one of the two following workarounds, the first one being preferred. 1) inherit flag-o-matic append-flags -DBOOST_FILESYSTEM_VERSION=2 2) If the build system doesn't respect CXXFLAGS, patch the source to contain #define BOOST_FILESYSTEM_VERSION 2 before any boost.filesystem header is included. Note that support for version 2 will not be included in boost-1.48 and later and upstream should migrate to version 3.
Boost.Filesystem Version 2 finally has been actually deleted in Boost 1.50.
Closing since all dependent bugs are closed.