Detect whether we actually have the long double type, before looking for those functions.

(closes issue #15017)
 Reported by: tzafrir
 Patches: 
       20090916__issue15017.diff.txt uploaded by tilghman (license 14)
 Tested by: tzafrir


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@219007 65c4cc65-6c06-0410-ace0-fbb531ad65f3