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/.gitignore

@@ -0,0 +1,2 @@
+*.out
+whichmake

t/203-make/Makefile

@@ -0,0 +1,8 @@
+default:
+
+x:
+	+redo x1.out x2.out
+
+y:
+	+redo x
+	+redo y1.out y2.out

t/203-make/all.do

@@ -0,0 +1,20 @@
+exec >&2
+redo-ifchange whichmake
+
+run() {
+    rm -f *.out
+    ./whichmake y
+
+    rm -f *.out
+    ./whichmake -j10 y
+
+    rm -f *.out
+    redo y
+
+    rm -f *.out
+    redo -j10 y
+}
+
+run
+. ./wipe-redo.sh
+run

t/203-make/clean.do

@@ -0,0 +1 @@
+rm -f *~ .*~ *.out whichmake

t/203-make/default.out.do

@@ -0,0 +1 @@
+echo $1 >$3

t/203-make/whichmake.do

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+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
+

t/203-make/wipe-redo.sh

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+vars=$(
+	env | {
+		IFS="="
+		while read key value; do
+			if [ "$key" != "${key#REDO}" ] ||
+			   [ "$key" != "${key#MAKE}" ]; then
+				echo "$key"
+			fi
+		done
+	}
+)
+echo "Wiping vars:" $vars >&2
+unset $vars

t/203-make/x.do

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+./whichmake x >&2

t/203-make/y.do

@@ -0,0 +1 @@
+./whichmake y >&2