BUILD: makefile: work around another bug in make 3.80

BUILD: makefile: work around another bug in make 3.80

GNU make 3.80 has an issue with calls to functions inside an if block,
which is just what we recently introduced to simplify the targets
declaration. The fix is easy, it simply consists in assigning the
command to a variable inside the if block and evaluating this command
after the block. This also makes the code slightly more readable so we
can keep compatibility with 3.80 for now.

No backport is needed.