VS: Do not use native unity builds on VS 2017 versions less than 15.8
VS 2017 version 15.8 introduced support for unity builds implemented
in part by the $(VCTargetsPath)/Microsoft.Cpp.Unity.targets
file.
Do not enable unity builds on VS 15 versions that do not have that file.
Fixes: #20284 (closed)
Backport: release
Edited by Brad King