Actually use _ASTLDFLAGS in the main/ and channels/ Makefiles.

They were previously passed correctly, but they simply weren't used.  This
caused issues with various platforms whose builds needed to pass special
linker flags via the configure script.

(closes issue #16596)
 Reported by: pprindeville
 Patches: 
       asterisk-1.6-astldflags.patch uploaded by pprindeville (license 347)
 Tested by: tilghman


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@245578 65c4cc65-6c06-0410-ace0-fbb531ad65f3