Draft: Optional CMake target dependencies

Convert target dependencies to cmake generator expressions. If a downstream project does not find a dependency, the generator expression will generate an empty string. If the dependency is found, the generator expression will fill the corresponding dependencies for that target. This has the effect to make dependencies optional from the point of view of the upstream module. Notice that these generator expressions include the corresponding compiler definitions, thus, removing most of the #define problems we have with #234 (closed).

Note: this is an experiment and is not yet fully tested!