gbuild: MSVC: disable warning C4373 again

Making our code acceptable for ancient compilers that we don't use and
that don't actually support C++ is a complete waste of time.

(partially reverts 856b7f5f783471388cd811d92172cc1d2da7e331)

Change-Id: I26a1a67d8c4beb31d6c0c301a56a948552683422