pgsql: pg_upgrade: Avoid check target accidentally breaking make's--ou

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/5af2e976d72aa345337596cc986237c57e1146b2

Modified Files
--------------
src/bin/pg_upgrade/Makefile | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)