VS: Avoid VS 2017 toolset default use of /FC flag

VS 2017 toolsets now use this flag by default if `UseFullPaths` is
not explicitly set to `false` in the .vcxproj file.  Since there is
no negative form of this flag there is no way for projects to turn
it off through our flag map.  Also, the Makefile and Ninja generators
do not add this flag unless it is explicitly specified by the project.

Teach our generator to set `UseFullPaths` to `false` in VS 2017
unless the project or use has explicitly specified `/FC`.

Fixes: #18261
Status Job ID Name Coverage
  External
passed cdash

passed cmake-terfin-windows-release-vs2017

passed ghostflow-branch-check-master

passed ghostflow-commit-check

passed ghostflow-stager

passed cmake-terfin-windows-release-vs2017

passed cmake-terfin-windows-release-vs2017

01:07:53

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager