Multiplatform Makefile Update

Paul Belanger pointed out that using sed in the Makefile is an issue with multiple platforms.  We are cleaning up the Doxygen config as a following step so I just switched the sed inplace changes to be an echo append instead.

(issue ASTERISK-20259)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@374963 65c4cc65-6c06-0410-ace0-fbb531ad65f3