instead of trying to forcibly load res_agi when app_stack is loaded (even if...

instead of trying to forcibly load res_agi when app_stack is loaded (even if the administrator didn't want it loaded), use GCC weak symbols to determine whether it was loaded already or not; if it was loaded, then use it.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@153709 65c4cc65-6c06-0410-ace0-fbb531ad65f3