Heads up. Upgrading the autotools and expat versions

Folks,

There are some bugs in the expat version we're using in libwww. I've
been working on upgrading to the latest version. 

I profited from this opportunity to also upgrade to the latest
autotools: autoconf, libtool, automake, autoheaders.

I'm removing some compiler warnings that came from this change. I'll
announce you when I make my commit. It may be that because of this
upgrade there are configure probles in some platforms (some macros
disappeared, others changed). I did my best. 

I'd appreciate your help testing the new configure and, if you have a
problem afterwards, please report it to the list ASAP so that I can fix 
it. After a while, my attention will wander again elsewhere :)

-jose