Add SHELL to fixinc/Makefile.in

[Laurynas Biveinis]

> > If SHELL is not set to /bin/sh, command.com is
> > used under DOS. Which is disaster, really.
> 
> > OK for head/branch?
> 
> No.  It should take whatever the superior Makefile
> is using.  

Sorry, I wasn't clear in my first message. Let me explain -
this patch takes effect when one says `make something' in
fixinc subdir and not when it is called from gcc Makefile.
In that case we don't inherit shell setting and we need
sensible definition. If it is called from gcc Makefile, then
its setting will override this default.

Does it make any sense now?

Laurynas