A brief exercise using GNU Make:

submake :
  ./subMake.sh

#!/bin/sh
make -f Submakefile

STUFF = a b c d e

all : $(STUFF)

$(STUFF) :
  @echo making $@
  @sleep 1

Run ‘make’. You’ll see a, b, c, d, and e get made serially, one per second. Want to parallelize? You’d think ‘make -j5′ would do it, right? You’d be wrong. The Submakefile complains that there is no jobserver available, and still makes them serially. The reason is that the toplevel Makefile doesn’t “think” that subMake.sh is a sub-make, and doesn’t pass along the necessary magic environment variables and file descriptors for the jobserver stuff to work. But, by merely adding a COMMENT:

submake :
  ./subMake.sh # $(MAKE)

IT WORKS!