actually check for a function present in libiconv (don't know how this test could...

actually check for a function present in libiconv (don't know how this test could have worked before) and don't do the check on Linux/GNU systems because libiconv is not present there and attempting to link with '-liconv' always fails (it's not necessary as the iconv functionality is always available)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@95817 65c4cc65-6c06-0410-ace0-fbb531ad65f3