[RFC] Add expat to the GDB sources

[Daniel Jacobowitz <drow@false.org>]

At the beginning of the year, I proposed adding an XML parsing library to
GDB.  There was some discussion, and a few alternatives were proposed which
would accomplish the same goals, but neither substantially better nor
substantially worse.  Since then, we've fleshed out the project the XML
descriptions were originally intended for (targets which can inform the
debugger about their registers), and added another (memory maps, used to
control flash writing).

I chose expat as the C XML library to use in GDB.  It's a bit frustrating
(it's a stream-oriented binding, and I find DOM (tree-oriented) bindings
much more natural), but it's also useful and of a reasonable size.  The most
logical candidate for a DOM binding would have been libxml2, and the source
for that is excessive to include in GDB.

Here is a patch to add an expat directory to the src repository, gdb builds,
and gdb release tarballs.  Not shown is expat itself.

Any comments?  I will hold off on this patch for a little while, at least
until one of the expat clients is ready.

-- 
Daniel Jacobowitz
CodeSourcery