Fail gracefully if XAudio2_8.dll is not present.

[rhdunn]

Use LoadLibrary/GetProcAddress instead of linking to xaudio2.lib in create_xaudio2_object to support returning NULL if the XAudio2 DLL is not present.

See rhdunn/espeak#98.

References:

1. https://msdn.microsoft.com/en-us/library/windows/desktop/ee415802(v=vs.85).aspx -- XAudio2 versions
2. https://blogs.msdn.microsoft.com/chuckw/2012/04/02/xaudio2-and-windows-8/

[mse2]

Another option is to use the waveOUT API which is available on older Windows versions too.
https://gitlab.com/mseide-msegui/pcaudiolib/blob/mse/src/waveout.c
https://gitlab.com/mseide-msegui/pcaudiolib/blob/mse/src/audio.c
It will be activated by the NO_XAUDIO2 define.
Binaries are here:
https://sourceforge.net/projects/mseuniverse/files/eSpeakNG/
rename "libespeak-ngwo.dll" to "libespeak-ng.dll" if you want to use it with "espeak-ng.exe".