mavlink/qgroundcontrol — pull request #486 by Susurrus (2014-01-29 21:26:41+00:00)
Title: Make speech synthesis support optional (version 2)

Added a #define flag for disabling speech support with `DEFINES+=DISABLE_SPEECH`.
Speech support is automatically detected on Linux and disabled (with output message to user) if missing.
Mac is just assumed to have support as it's available in 10.6+. There doesn't appear to be a way to check the Mac version within qmake.