Added SCons build files for external flite library

Added SCons build files for external flite library
    SConstruct
    external/flite/SConscript                 (+)
    external/flite/lang/SConscript            (+)
    external/flite/lang/cmu_us_awb/SConscript (+)
    external/flite/lang/cmulex/SConscript     (+)
    external/flite/lang/usenglish/SConscript  (+)
    external/flite/main/SConscript            (+)
    external/flite/sdl2-cross-changes.md
    external/flite/src/SConscript             (+)
    external/flite/src/audio/SConscript       (+)
    external/flite/src/cg/SConscript          (+)
    external/flite/src/hrg/SConscript         (+)
    external/flite/src/lexicon/SConscript     (+)
    external/flite/src/regex/SConscript       (+)
    external/flite/src/speech/SConscript      (+)
    external/flite/src/stats/SConscript       (+)
    external/flite/src/synth/SConscript       (+)
    external/flite/src/utils/SConscript       (+)
    external/flite/src/wavesynth/SConscript   (+)