Merged revisions 242520 via svnmerge from

https://origsvn.digium.com/svn/asterisk/branches/1.4

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  r242520 | tilghman | 2010-01-24 00:33:01 -0600 (Sun, 24 Jan 2010) | 8 lines
  
  Only rebuild bison and flex source files on demand, if bison and flex are detected by the configure script.
  
  Changed after discussion on the -dev list about possible unnecessary build
  failures, due to checkouts/untars causing these special source files to
  possibly be newer than their resulting C files.  This should additionally
  ensure that nobody need learn about extra Makefile arguments to ensure the
  proper files get rebuilt when changes are made to these special source files.
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git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@242521 65c4cc65-6c06-0410-ace0-fbb531ad65f3