Split the makefile into more files and introduce a BUILD_PREFIX variable to be used instead of .build-cache, now defaulting to artifacts so that it is more easily found. Adjust .gitlab-ci.yml to set the prefix to the empty string, and also to use 8 job slots, which should speed things up a little.
Closes #36 (closed)
Closes #2 (closed)
Closes #190 (closed)