Modify build systems so that it automatically builds files in src folder

[SanderMertens]

Makefiles include a full list of source files that have to be compiled for a particular artefact. This is tedious, every time a file is added or removed the makefile must be changed as well. Modify build system so that makefiles don't have to explicitly provide this list of files. Instead, just include all files in source folder.

This will greatly simplify issue #44.