Blender Git Loki

CMake: Have CMake Control the C++ version.

We were manually setting the compiler flags
for C++17 support for this previously. CMake
can do this for us in a uniform way without
having to worry about compiler specifics.

Setting these flags manually somehow brought
out some unwanted behaviour (CMake switching
back to C++14) in the nightly CMake builds.

Unsure if that's a CMake bug or planned
new behaviour for future version, but best
to play it safe.

These flags are supported since CMake 3.1
so should not break anything.

Reviewed by: Campbell Barton

Differential Revision: https://developer.blender.org/D11891