[CMake] Visual SourceSafe and Relative Paths for MSVC

[Sylvain Benner]

> It seems that although when I add a file to a library with a relative 
> path, the resulting MSVC project contains the file with an absolute 
> path. While it works for compiling, it doesn’t work for the Microsoft 
> visual source safe plug-in in Visual Studio. Does Cmake support 
> Microsoft’s visual source safe plug-in in Visual Studio? Is there any 
> reason not to use relative paths?
>
Try using

SET(CMAKE_USE_RELATIVE_PATHS 1)

--Sylvain