Because of a change in VS toolset, missing option <UseFullPaths>
is no longer interpreted as "don't pass /FC to the compiler", but
rather as "pass /FC to the compiler". This is problematic, because
/FC not only changes how much of the path is reporter by the compiler
(e.g. in `__FILE__` macro), but it also lower cases the path.
This lower-casing of the path broke our approval tests for VS2017
about 5 months ago.
Using CMake 3.13 (not yet released) would also let us fix it, but
for now we use a vcxproj.user file that is merged with the main project
and explicitly disables `/FC`.
