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.

install.do

@@ -1,5 +1,5 @@
 exec >&2
-redo-ifchange Documentation/all
+redo-ifchange _all
 
 : ${INSTALL:=install}
 : ${DESTDIR:=}
@@ -28,6 +28,10 @@ for d in *.py; do
 done
 python -mcompileall $LIBDIR
 
+# 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.
+cp -R redo-sh/sh $LIBDIR/sh
+
 # binaries
 for dd in redo*.py; do
 	d=$(basename $dd .py)