Fix spacing arround assignment in spec.gmk.in

For some reason, we have not been consistent with using spaces around the assignment operators (`:=` and `=`) in spec.gmk.in. This has annoyed me for a long time.

When making this change, I noticed that there are a lot of late evaluation assignments (= instead of :=) that should not be there. To not confuse the code review, I will fix these separately. Thus this change will only change whitespace.