We got a report yesterday on the FEniCS mailing list about a problem with Boost (build.log). It turns out that the current Boost version in hashstack, 1.55.0, does not work with GCC 5 (also reported here). Updating Boost to 1.56.0 or later will solve this problem as this is fixed upstream, however, for now I will create a pull request with the patch that fixes the problem.