xpath functions

Here's a clean version of the xpath changes. Please reject the earlier pull update-xml request. This keeps the older thirdparty libxml library (updating to a newer version is left for a separate request) and implements two new xpath functions. Changes made to linux Makefiles only at this point. From previous experimentation there are some similar changes that need to be made to the XCode project files, but I'm leaving them for the future because I'm not sitting at a Mac right now and I don't want to guess about them.