Blender Git Loki

MSVC: Fix cmake warnings with MSVC updates

CMake caches the location of the CRT runtime in the
MSVC_REDIST_DIR variable, and uses it to copy the required
dll's during the install phase.

This variable is only initialized when it does not exist.
Leading to issues when compiler updates are installed and
the compiler version slightly changes, cmake still looks
in the old location for the runtime and warns about the
files not existing.

This change fixes the issue by checking if the redist dir
exists and if not unsets it so InstallRequiredSystemLibraries
can have another go at figuring out where they live.