Fix macOS crackling/stuttering related to SDL buffer, use Portaudio for playback instead of SDL

[adroste]

For some reason the use of SDLPlayback in combination with PortaudioInput causes weird playback issues on my Mac.
The issue can be partly resolved by using an extremely large buffer size for the SDL playback (8196 and above).

If I use
PortaudioPlayback + PortaudioInput -> no issues (low latency < 2ms possible with a few code changes)
SDLPlayback + SDLInput -> no issues (low latency <2ms possible with a few code changes)

Therefore the following commit fixes all my issues. What was the reason for using SDLPlayback when Portaudio is available?

Fixes #573

[s09bQ5]

I feel uncomfortable with changing the default for all *NIX platforms if it so far appears to be a problem only on macOS.

I think the goal has always been to get rid of PortAudio since we were already linking to SDL for graphics, keyboard, and mouse. We didn't use SDL for recording audio in the past because the API didn't support it. Now it does, which recently led me to add the --without-portaudio configure switch.

[adroste]

If the goal is to get rid of PortAudio then the fix should be to don't define "HavePortaudio" on Darwin.
However, PortaudioInput+PortaudioPlayback and SDLInput+SDLPlayback work fine, mixing them breaks things (for whatever reason).

Edit: The behavior of Portaudio is less prone to config issues. Portaudio seems to set the buffers/latency to a stable minimum, whereas SDL just uses your configured buffer size causing crackling/stuttering or even no audio if set too low.

@s09bQ5 Before I make another commit, what would be your preferred way to remove Portaudio on macOS? Just undef the HavePortaudio if Darwin?

[s09bQ5]

I'd say the best way is to change the default value of with_portaudio in configure.ac from "check" to "no" if configure is run on macOS.