Boost 1.68 has a bug in its type_traits where it does not include
the right header for the is_assignable macro when building with clang
We got two options here
- Force all platforms to upgrade to boost, this is rumoured to be fixed in 1.69  but I have yet to validate this.
- Insert this eye sore into quadraflow's config.hpp
Given changing boost versions tends to be 'unpopular' and there are currently very few upsides of doing that, this patch implements 2