Handle .do files that start with "#!/" to specify an explicit interpreter.

Now you can have your .do files interpreted by whatever interpreter you
want.

README.md

@@ -437,15 +437,28 @@ changed, you can run `redo-ifchange target` instead.
 
 # Can my .do files be written in a language other than sh?
 
-FIXME: Not presently.  In theory, we could support starting your .do files
-with the magic "#!/" sequence (eg. #!/usr/bin/python) and then using that
-shell to run your .do script.  But that opens new problems, like figuring
-out what is the equivalent of the `redo-ifchange` command in python.  Do you
-just run it in a subprocess?  That might be unnecessarily slow.  And so on.
+Yes.  If the first line of your .do file starts with the
+magic "#!/" sequence (eg. `#!/usr/bin/python`), then redo
+will execute your script using that particular interpreter.
 
-Right now, `redo` explicitly runs `sh filename.do`.  The main reasons for
-this are to make the #!/ line optional, and so you don't have to remember to
-`chmod +x` your .do files.
+Note that this is slightly different from normal Unix
+execution semantics: redo never execs your script directly;
+it only looks for the "#!/" line.  The main reason for this
+is so that your .do scripts don't have to be marked
+executable (chmod +x).  Executable .do scripts would
+suggest to users that they should run them directly, and
+they shouldn't; .do scripts should always be executed
+inside an instance of redo, so that dependencies can be
+tracked correctly.
+
+WARNING: If your .do script *is* written in Unix sh, we
+recommend *not* including the `#!/bin/sh` line.  That's
+because there are many variations of /bin/sh, and not all
+of them are POSIX compliant.  redo tries pretty hard to
+find a good default shell that will be "as POSIXy as
+possible," and if you override it using #!/bin/sh, you lose
+this benefit and you'll have to worry more about
+portability.
 
 
 # Can a single .do script generate multiple outputs?