autoconf 2.5 applied

I've applied the autoconf patch. There where a couple of last-minute
issues..

Henrik,
I'm not sure I've done the 'right thing' with bootstrap, but it works
for me on squid-cache.org and my debian sid boxen.

Also, there was a conflict with redefining FD_SETSIZE when sys/types was
included on freeBSD. I wasn't sure what was the right thing to do for
xBSD, so I simply put a
#ifdef FD_SETSIZE
#undef FD_SETSIZE
#endif
in squid.h for now.

Cheers,
Rob