This error message means that there is an endless loop in how
your projects depend on each other, making it impossible to
correctly build. For example, compiling project A requires the
build output of project B, and compiling project B also requires A.
Eclipse sometimes gives these error by mistake, so try doing
a clean of all the projects in your workspace.
If that doesn't fix it, go to the project's Properties, and
see if your Library Paths create an endless loop. If not, you can
also check the more general list under Properties > Project