bidding | The server side of a bidding app | Runtime Evironment library

The answer is strictly due to make's rules and has nothing to do with any of the other tags here (I'll remove all the others). The makefile that doesn't work says:

• to build gitbranch-foo, we must build git-spawn-branch
• to build gitbranch-bar, we must build git-spawn-branch

(plus of course the code for building git-spawn-branch itself and the other associated stuff, but let's stop here).

You then run make, telling it to build both gitbranch-foo and gitbranch-bar.

Make picks one of these to build—gitbranch-foo, in this case—and begins building. Oh hey, says make to itself, this needs git-spawn-branch to be built, and we have not done that yet. Off goes make, which builds git-spawn-branch according to the rules. Now it's built! Make can go back to building gitbranch-foo. This executes the remaining recipe, which is empty.

If parallel builds are allowed, make can also now begin building gitbranch-bar while gitbranch-foo builds. If not, we wait for gitbranch-foo to be fully built at this point. Either way we very quickly get around to buildling gitbranch-bar. Hm, says make to itself, this needs git-spawn-branch to be built ... but fortunately, I have done that already! Onward! Let's make the rest of gitbranch-bar now! That requires executing the empty recipe, which goes very quickly.

Make is now done. It has built everything required.

(The makefile that works uses $call sensibly, directly from each rule, so that the commands that $call expands to are required to be run for each target, rather than hidden away in a third target that can be built just once and never needs to be run any further.)

(Note that the gmake documentation mentions that a .PHONY rule is run every time. This means once per invocation of make, not once per invocation of the rule.)