BSD make can run parallel more aggressively than GNU make. It communicate
with other make process through -J option in MAKEFLAGS environment variable
to notify a build failure happend in an other pararell make process.
https://www.freebsd.org/cgi/man.cgi?make
It usually works well but ext/-test-/cxxanyargs/Makefile has two targets
which are expected to fail (failure.o and failurem1.o).
Additional note:
To test and debug this issue, following command will speed up it.
`make -f exts.mk -j8 clean all`
