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

1. Force all platforms to upgrade to boost, this is rumoured to be fixed in 1.69 [1] but I have yet to validate this.

2. 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

[1] http://boost.2283326.n4.nabble.com/quot-is-assignable-quot-compile-error-using-Boost-1-68-LLVM-Clang-Windows-td4704917.html