CMake preprocessor definitions for libraries not working correctly

[lobis]

When a library is set to be compiled, a preprocessor definition is defined (see https://github.com/rest-for-physics/framework/blob/master/cmake/MacroRootDict.cmake#L523) to tell other libraries that the corresponding library exists.

For example if the detector library is found, REST_DetectorLib will be defined as a predecesor directive and this can be used in another library to add some code that would otherwise fail.

However the current implementation is not working correctly due to some bug, as pointed out by @KonradAltenmueller.