As it currently stands, boost's serialization library is unable to be compiled on mingw-w64. As such, I've made some changes to the ebuild in order to separate that library into a separate use flag, so that one can at least use the other libraries on mingw-w64 until upstream boost gets it fixed.
boost-1.66.0 compiles without errors on mingw-w64 (with some ebuild adjustments). Will wait it out until then.