The main CMakeLists.txt specifies C++17, quadriflow tries to add C++14
flags leading to the following warnings when building with MSVC
Command line warning D9025 : overriding '/std:c++17' with '/std:c++14`
This change removes the C++14 flags, and fixes a build error caused
by the removal of `std::unary_function` in C++17 in the .obj loader
(which isn't used by blender)
Reviewed By: zeddb
Differential Revision: https://developer.blender.org/D8720
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
