I cannot find a way such that the project becomes executable with just an added empty window. Neither by adding the mentioned Geany build commands (This gives me "undefined reference" errors, where there weren't any before), nor by adjusting the makefile so that it looks like the one for the HelloWorld example, with added different parameter for compiling each .o files. I think the problem is, that in these example programs, there aren't as many files linked together as in our project and I might miss some parameters.
Is there an example of how the makefile should look like when there are more than just one or two source/header files?