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. Temporarily work
around it until we upgrade to a newer Boost version that fixes it.
Differential Revision: https://developer.blender.org/D5962
The config.h file is autogenerated during compile by a 3rd party library
quadriflow uses. Now we put this file in the build directory to avoid
adding this to the git repository in the future.