pg_upgrade: Avoid check target accidentally breaking make's --output-sync.
When $(MAKE) is present in a rule, make assumes that target is a
submake, and it doesn't need to buffer its output. But in this case
it's a shell script that needs buffered output. Avoid that heuristic,
by referring to $(MAKE) via an indirection.
Discussion: https://postgr.es/m/
20190521004717.qsktdsugj3shagco@alap3.anarazel.de