Produce straight-line shell code

[ndmitchell]

If a build system is primarily cmd actions, then these could be captured in a full -j1 build, and written out as a .sh/.bat file.

[ndmitchell]

@yminsky just pointed out I could generate a makefile now I have unrolled the dependencies, which is a far better idea.

[ndmitchell]

Could also produce Ninja or Fastbuild output, since that's not really any harder. see #614

[yminsky]

In a case of amusingly close timing, a rather similar feature is in progress for Dune: ocaml/dune#1293