Use the SHELL variable instead of assuming 'sh'.

This is what GNU make does.  If SHELL isn't defined, we still fall back to
calling sh.

Rumour has it that Google has some kind of build system that can be
massively distributed if you just set SHELL to the right program; maybe
it'll work with redo now.  (Of course it won't do you any good until we
implement parallel builds...)

redo.py

@@ -85,7 +85,8 @@ def build(t):
     tmpname = '%s.redo.tmp' % t
     unlink(tmpname)
     f = open(tmpname, 'w+')
-    argv = ['sh', '-e', os.path.basename(dofile),
+    argv = [os.environ.get('SHELL', 'sh'), '-e',
+            os.path.basename(dofile),
             os.path.basename(t), 'FIXME', os.path.basename(tmpname)]
     if vars.VERBOSE:
         argv[1] += 'v'