Ignore expected errors on compiling C++ source [Bug #16331]

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`

ext/-test-/cxxanyargs/depend

@@ -1,6 +1,7 @@
 $(TARGET_SO) $(STATIC_LIB): $(FAILURES:.cpp=.failed)
 
 .SUFFIXES: .failed
+.IGNORE: failure.o failurem1.o
 
 .cpp.failed:
 	$(Q)$(RUBY) -rfileutils \