Added flite library for text to speech synthesis

Added flite library for text to speech synthesis

Added a heavily stripped down version of flite-1.4 from
http://www.festvox.org/flite/

To see changes from original source repository see:
    external/flite/sdl2-cross-changes.md   (+)

Other changes:
    README.md
    external/flite/ACKNOWLEDGEMENTS (+)
    external/flite/COPYING          (+)
    external/flite/README           (+)
    external/flite/include/*        (+)
    external/flite/lang/*           (+)
    external/flite/main/*           (+)
    external/flite/src/*            (+)