Version Control With Subversion Source

When compiling the svn book, handle the case where "svnversion ."
doesn't work (i.e. we don't have an svnversion executable in our $PATH
or we aren't building inside a checked-out working copy).

* doc/book/Makefile (BOOK_VERSION_SOURCE): Check to see if "svnversion
." produces an error before trying to use its output. Provide a
fallback for the version.xml entity if this doesn't work.

* doc/book/book.xml: Let version.xml provide the complete subtitle
text.