Add some extra checking to help out with a potential error when trying to

run "make asterisk.pdf" when not all of the right packages are installed.

(closes issue #10763)
Reported by: Corydon76
Patches:
      20070919__bug10763.diff.txt uploaded by Corydon76 (license 14)
Tested by: Corydon76


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@98454 65c4cc65-6c06-0410-ace0-fbb531ad65f3