Add debugging code for examining query strings.

Add debugging code for examining query strings.

It's nice that Xapian provides a little function to print a textual
representation of the entire query tree. So now, if you compile
like so:

	make CFLAGS=-DDEBUG_QUERY

then you get a nice output of the query string received by the query
module, and the final query actually being sent to Xapian.