aggregate.make will always do a serial build in older versions of gnustep-make. In newer versions, it will do a serial build by default, but will do a parallel build if you set
GNUSTEP_USE_PARALLEL_AGGREGATE = yes
So, here is how to build a set of subdirectories in serial order -
include $(GNUSTEP_MAKEFILES)/common.make SUBPROJECTS = Source Tools include $(GNUSTEP_MAKEFILES)/aggregate.make
and here is how to build them in parallel oder -
include $(GNUSTEP_MAKEFILES)/common.make SUBPROJECTS = Source Tools GNUSTEP_USE_PARALLEL_AGGREGATE = yes include $(GNUSTEP_MAKEFILES)/aggregate.make
This last example will simply do a parallel build on newer gnustep-makes, and a serial build on older ones that don't support it.
aggregate.make will be slowly phased out in the following years, so if you don't need to support older versions of gnustep-make, you could simply use serial-subdirectories.make and parallel-subdirectories.make.