   Sharing Code Path with Multiple Projects in VS 2008
  I wrote a game engine in OpenGL. It is just source (headers and C++ source files), and it is not compiled into a static library or anything like that since I am still making changes on it constantly.I need to use this engine in about 4 projects at one time. All of these projects are in a single solution in Visual Studio. The only problem is that if I want to use the engine, I would have to copy the engine code 4 times into each local directory for each project.What I want to do is have all my engine source in one folder, and have all 4 projects reference that folder in Visual Studio so that I do not have to write the relative path of each engine header file I use in my project. Is this possible to do?   
  All you need to do is add the engine source files to each project.  You don't have to actually copy the files, since all four projects can add files from the same physical location.  However all things considered this isn't a good idea, since you have to duplicate references to the engine source four (or more) times, and compile the same file four times every time it needs to be compiled.  Ironically, all that extra compiling would be a lot worse than just using a static/dynamic library in the first place, especially since you mentioned the engine is constantly changing.   