initial patch to install libiberty headers

[DJ Delorie]

> > I don't know of any *serious* gotchas, but be aware that libiberty is
> > one of the few packages built *twice* in each "monolithic" build -
>                                                 ^^^^^^^^^^^^
> As opposed to...?  I'm missing background here.

As opposed to just building gcc, where it's really only used to build
host programs.  Libiberty is also used by binutils, gdb, newlib,
cygwin, and possibly others.  It's possible to build all those
packages from a single source tree, and when you do that, libiberty
really gets a workout - especially when building a cygwin native
toolchain.