Blender Git Loki

MSVC: Set proper flags for C++17 support.

MSVC already builds with the /std:c++17 flag but for
'reasons' [1] MSVC still gives the wrong value for the
__cplusplus define.

This change sets an additional cxx flag on supported
compilers to allow the compiler properly identify
C++17 support.

This resolves 2 warnings coming out of bullet about
the register keyword being deprecated.

[1] https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/