linux speech synthesis: switch to eSpeak

Conflicts:
	QGCExternalLibs.pri
	src/GAudioOutput.cc
Merge remote-tracking branch 'upstream/master' into espeak

Conflicts:
	QGCExternalLibs.pri
	src/GAudioOutput.cc
	src/GAudioOutput.h
remove alsa asound library dependency comment
fix merge errors and unused-variable error
espeak: cleanup
Readme.md: replace flite with espeak
espeak: remove c style zero initialize
espeak: remove comment
espeak: update readme
Merge pull request #471 from thomasgubler/espeak

linux speech synthesis: switch to eSpeak