Sometimes you want something like a symlink but isn't acutally a symlink so you can maintain cross platform support, In that case I do something like this:
You can then create a symlink file with the following syntax:
sphere_orbiter -> ../single_player/src/system_logic/sphere_orbiter
random_vector -> ../single_player/src/system_logic/random_vector
sphere_orbiter -> ../server/src/system_logic/sphere_orbiter
random_vector -> ../server/src/system_logic/random_vector
...