[Doc] Generate XML output with Doxygen

This output will be useful to generate links from the website pages to
the C++ documentation; see Cantera/cantera-website#16

.travis.yml

@@ -64,6 +64,7 @@ script: |
           --rsh='ssh -i ./doc/ctdeploy_key'
           --exclude='*.map'
           --exclude='*.md5'
+          --exclude='/doxygen/xml'
           --delete
           --delete-excluded
         )

doc/doxygen/Doxyfile

@@ -1163,7 +1163,7 @@ MAN_LINKS              = NO
 # generate an XML file that captures the structure of
 # the code including all documentation.
 
-GENERATE_XML           = NO
+GENERATE_XML           = YES
 
 # The XML_OUTPUT tag is used to specify where the XML pages will be put.
 # If a relative path is entered the value of OUTPUT_DIRECTORY will be