Previously it was used only on debug builds, but recent
additions to variant_call.cpp have made that unit too
large even in release_debug.
It was originally set to debug only because using it
breaks mingw, so this only reinstates it for MSVC where
it shouldn't have any drawbacks (famous last words).
Test-Information:
Builds for me with
scons -j 40 platform=windows target=release_debug
and my game then runs from it.
(cherry picked from commit d8900e0020b21d449069ef2a301253371fb70a21)
