Version Control With Subversion Source

shouldn't. It was always updating book-html-chunk, and never updating the
embedded revision number.

* doc/book/Makefile
Re-check the output of "svnversion" every time, but only update version.xml
if the revision number has changed. (Previously version.xml was never
updated unless it had been deleted.)
Only rebuild book-html-chunk if it is older than its (main) source files.
(Previously it was re-built every time.)
The image files are now dependencies of most targets.