The test for __cxa_demangle(), which is used by Dune::className() to
demangle type names, doesn't work on my clang 3.6. Apparently, clang
requires the user to include <typeinfo> before <cxxabi.h>. While
classname.hh already does this, the build system tests don't, causing the
feature to get disabled on clang.

Fixed by adding the include to the build system tests.