fix up CMake target dependencies

On systems with no Eigen3 or fftw3, triggering a parallel build (i.e. `make -j<njobs>`) the build process is likely to fail as the examples and/or tests are build before the external projects (`Eigen3` and `fftw3`) are finished building. This fixes that problem by explicitly adding these two to the list of dependencies of the tests and examples, which need them.