Add t/example, a basic example build environment suitable for a tutorial.

t/example/CC.do

@@ -0,0 +1,10 @@
+redo-ifchange config.sh
+. ./config.sh
+cat <<-EOF
+	redo-ifchange \$1.c
+	gcc -MD -MF \$3.deps.tmp -o \$3 -c \$1.c
+	DEPS=\$(sed -e "s/^\$3://" -e 's/\\\\//g' <\$3.deps.tmp)
+	rm -f \$3.deps.tmp
+	redo-ifchange \$DEPS
+EOF
+chmod +x $3