Blender Git Loki

Cleanup: Fix build warning with MSVC and OSL

OSL requires RTTI to be off, this is done with the /GR- flag for
MSVC, however /GR is in the default CXX flags leading to warning

D9025 : overriding '/GR' with '/GR-'

which cannot be suppressed.

/GR is on by default and this flag is not required, so removing
it from the default CXX flags makes it possible later use /GR-
without generating warnings.