jwack: _cheatfds error when run from toplevel make -j.

Also added a new unit test to confirm that 'make' behaviour works as
expected, with and without parallelism.

t/203-make/whichmake.do

@@ -0,0 +1,16 @@
+if type gmake >/dev/null 2>/dev/null; then
+	make=gmake
+elif type make >/dev/null 2>/dev/null; then
+	make=make
+else
+	# No make installed?  That's okay, this test
+	# isn't *that* important.
+	make=:
+fi
+
+cat >$3 <<-EOF
+	#!/bin/sh
+	$make "\$@"
+EOF
+chmod a+x $3
+