Math: fixed test compilation with libc++.
Math: fixed test compilation with libc++.

Apparently std::forward() is constexpr only in C++14. Heh.

ashimidashajia/magnum,DerThorsten/magnum,ashimidashajia/magnum,ashimidashajia/magnum,MiUishadow/magnum,MiUishadow/magnum,DerThorsten/magnum,ashimidashajia/magnum,DerThorsten/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,DerThorsten/magnum,DerThorsten/magnum,MiUishadow/magnum,ashimidashajia/magnum