Creating new doxygen macro "\extref" to create page that lists

external libraries and URLs to these. Please help me add these
references.

We might want to create a similar macro "\linuxpackage" to list
the needed Linux packages in popular distributions.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@56647 65c4cc65-6c06-0410-ace0-fbb531ad65f3