Version Control With Subversion Source

build rules.

* tools/Makefile.base-rules
(epub): Depend on $(UNIFIED_XML_TARGET) rather than
book/xml-book.xml explicitly.
(UNIFIED_XML_TARGET): Was book/svn-book.xml explicitly.
(clean): Remove the UNIFIED_XML_TARGET, too.

* tools/Makefile.base-vars
(UNIFIED_XML_TARGET): New.
(ALL_SOURCE): Filter UNIFIED_XML_TARGET, not $(DIR)/svn-book.xml
explicitly.