[O] Preserve tabs in tangled babel output

[Roger Mason]

Hello,

This code:

#+BEGIN_SRC makefile :tangle Makefile
  hello:  hello.f90
          gfortran hello.f90

  execute: hello
          ./a.out

  clean:
          rm -f a.out *.o
#+END_SRC

(stolen from John Kitchin) produces a Makefile with spaces instead of
tabs.  I created the content in makefile mode, with tabs, but emacs
untabifies the output.  How can I prevent this?

Thanks,
Roger
Org-mode 8.2.6