Automatically select a good shell instead of relying on /bin/sh.

This includes a fairly detailed test of various known shell bugs from the
autoconf docs.

The idea here is that if redo works on your system, you should be able to
rely on a *good* shell to run your .do files; you shouldn't have to work
around zillions of bugs like autoconf does.

redo-sh.do

@@ -0,0 +1,45 @@
+exec >&2
+redo-ifchange t/shelltest.od
+
+rm -rf $1.new $1/sh
+mkdir $1.new
+
+GOOD=
+WARN=
+
+for sh in dash sh ash ksh pdksh bash zsh busybox; do
+	printf "Testing %s... " "$sh"
+	FOUND=`which $sh` || { echo "missing"; continue; }
+	
+	# It's important for the file to actually be named 'sh'.  Some shells (like
+	# bash and zsh) only go into POSIX-compatible mode if they have that name.
+	# If they're not in POSIX-compatible mode, they'll fail the test.
+	rm -f $1.new/sh
+	ln -s $FOUND $1.new/sh
+	
+	set +e
+	( cd t && ../$1.new/sh shelltest.od >/dev/null 2>&1 )
+	RV=$?
+	set -e
+	
+	case $RV in
+		0) echo "good"; GOOD=$FOUND; break ;;
+		42) echo "warnings"; [ -n "$WARN" ] || WARN=$FOUND ;;
+		*) echo "failed" ;;
+	esac
+done
+
+rm -rf $1 $1.new $3
+
+if [ -n "$GOOD" ]; then
+	echo "Selected perfect shell: $GOOD"
+	mkdir $3
+	ln -s $GOOD $3/sh
+elif [ -n "$WARN" ]; then
+	echo "Selected mostly good shell: $WARN"
+	mkdir $3
+	ln -s $WARN $3/sh
+else
+	echo "No good shells found!  Maybe install dash, bash, or zsh."
+	exit 1
+fi