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/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