Allow xmllint to be used for XML docs validation.

xmllint seems to be more commonly available since it comes with libxml2.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@277703 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configure.ac

@@ -202,6 +202,7 @@ AC_PATH_PROG([CURL], [curl], :)
 AC_PATH_PROG([RUBBER], [rubber], :)
 AC_PATH_PROG([CATDVI], [catdvi], :)
 AC_PATH_PROG([KPATHSEA], [kpsewhich], :)
+AC_PATH_PROG([XMLLINT], [xmllint], :)
 AC_PATH_PROG([XMLSTARLET], [xmlstarlet], :)
 if test "${WGET}" != ":" ; then
   DOWNLOAD=${WGET}