Second round of cleanups for LibXML2 docs/examples

configure.am:

* Explicitly disallow --enable-rebuild-docs when builddir != srcdir, per
   what you said about needing to build docs with an in-source build

doc/Makefile.am:

* Ensure that xmlversion.h is in the source tree before running
   apibuild.py, to avoid generating an incomplete libxml2-api.xml

* Update the .PHONY target (forgot to do this earlier)

doc/devhelp/Makefile.am:

* Wrap the doc-generating rule in an "if REBUILD_DOCS" conditional so it
   doesn't cause trouble for regular users

* Added a handy-dandy "rebuild" target

doc/examples/index.py:

* NOTE: You need to run this script to regenerate the files it creates,
   and then commit the newly-updated files! The generated files currently
   in git master (e.g. doc/examples/Makefile.am) are out of date even
   before this patch!

* index.html really needs to be in EXTRA_DIST

* Wrap the doc-generating rules in an "if REBUILD_DOCS" conditional,
   because they shouldn't be active otherwise
diff --git a/doc/Makefile.am b/doc/Makefile.am
index e4b60a6..08c0b45 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -303,6 +303,7 @@
 	$(wildcard $(top_srcdir)/*.c)
 
 libxml2-api.xml libxml2-refs.xml ../libxml2.syms: apibuild.py symbols.xml syms.xsl checkapisym.xsl $(source_file_deps)
+	test -f $(top_srcdir)/include/libxml/xmlversion.h
 	(cd $(srcdir) && ./apibuild.py)
 	($(XSLTPROC) $(srcdir)/checkapisym.xsl $(srcdir)/libxml2-api.xml)
 	($(XSLTPROC) -o ../libxml2.syms $(srcdir)/syms.xsl $(srcdir)/symbols.xml)
@@ -346,4 +347,4 @@
 	-$(INSTALL) -m 0644 $(srcdir)/tutorial/images/callouts/*.* \
 		$(DESTDIR)$(HTML_DIR)/tutorial/images/callouts
 
-.PHONY : html xml templates scan
+.PHONY: docs api web wiki rebuild