Race to build pg_isolation_regress in "make -j check-world"

I've been enjoying the speed of parallel check-world, but I get spurious
failures from makefile race conditions.  Commit c66b438 fixed the simple ones.
More tricky is this problem of multiple "make" processes entering
src/test/regress concurrently, which causes failures like these:

  gcc: error: pg_regress.o: No such file or directory
  make[4]: *** [pg_isolation_regress] Error 1

  /bin/sh: ../../../src/test/isolation/pg_isolation_regress: Permission denied
  make -C test_extensions check
  make[2]: *** [check] Error 126
  make[2]: Leaving directory `/home/nm/src/pg/backbranch/10/src/test/isolation'

  /bin/sh: ../../../../src/test/isolation/pg_isolation_regress: Text file busy
  make[3]: *** [isolationcheck] Error 126
  make[3]: Leaving directory `/home/nm/src/pg/backbranch/10/src/test/modules/snapshot_too_old'

This is reproducible since commit 2038bf4 or earlier; "make -j check-world"
had worse problems before that era.  A workaround is to issue "make -j; make
-j -C src/test/isolation" before the check-world.  This problem doesn't affect
src/test/regress/pg_regress.  Every top-level "make" or "make install",
including temp-install, builds pg_regress.

I tried fixing this by building src/test/isolation at the same times we run
install-temp.  Naturally, that didn't help installcheck-world.  It also caused
multiple "make" processes to enter src/port concurrently.  I could fix both
check-world and installcheck-world with the attached hack of building
src/test/isolation during every top-level build or install.

The problem of multiple "make" processes in a directory (especially src/port)
shows up elsewhere.  In a cleaned tree, "make -j -C src/bin" or "make -j
installcheck-world" will do it.  For more-prominent use cases, src/Makefile
prevents this with ".NOTPARALLEL:" and building first the directories that are
frequent submake targets.  Perhaps we could fix the general problem with
directory locking; targets that call "$(MAKE) -C FOO" would first sleep until
FOO's lock is available.  That could be tricky to make robust.

For now, I propose back-patching the attached, sad hack.  Better ideas?

Thanks,
nm