[rfa] make clean should remove observe.h and observer.inc

[Mark Kettenis]

> Date: Thu, 24 Feb 2011 22:25:46 +0400
> From: Joel Brobecker <brobecker@adacore.com>
> 
> > That brings up another question.  What about <lang>-exp.c?  Technically
> > they are generated files.  Should make clean remove them?
> 
> I would say no - we check them in so to allow users to build
> GDB without requiring bison/flex. It would be problematic for
> them if they disappeared at a simple "make clean".

Indeed; "make clean" should not remove any files that are included in
the GDB release tarball.