[2853] Add Doxyfile-xml

doc/Doxyfile-xml

+# This is a doxygen configuration for generating XML output as well as HTML.
+#
+# Inherit everything from our default Doxyfile except GENERATE_XML, which
+# will be reset to YES
+
+@INCLUDE = Doxyfile
+GENERATE_XML           = YES

doc/Makefile.am

 SUBDIRS = guide
 
-EXTRA_DIST = version.ent.in differences.txt
+EXTRA_DIST = version.ent.in differences.txt Doxyfile Doxyfile-xml
 
 devel:
 	mkdir -p html