Add check for Doxygen

The autoconf configuration system had a test for DOT but not for Doxygen.  I added the test for Doxygen and did an overhaul of the Makefile check to a much simpler process.

(issue ASTERISK-20259)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@374940 65c4cc65-6c06-0410-ace0-fbb531ad65f3