Report errors on variable use in key

For https://bugzilla.gnome.org/show_bug.cgi?id=680938

Variables are forbidden for match or use values of keys
* libxslt/xsltutils.c libxslt/xsltutils.h: allows to add flags
  to XPath expression compilation
* libxslt/keys.c: add the XML_XPATH_NOVAR if defined (recent libxml2)
* libxslt/pattern.c: add a missing xpath.h include to make sure the
  defintiion is found there too