Enable ALSA only for Linux
Include stdlib.h for alloca() on OpenBSD
Include string.h for memset() on OpenBSD
Disable X11 joystick functions also for OpenBSD
Import sources for libao driver (unfinished, based on dummy driver)
Add 'ao' build option
Add ao/ subdir to drivers/SCsub
Use libao driver on X11 platform
Initialize/shutdown libao
Open/close libao audio device
Minor code formatting fixes in ao driver
Change 0 to NULL for ao_open_live() and device

It's NULL in other places.
Write audio samples to libao device
Check return value of ao_play()
Set libao sample format to 16 bits

Some plugins don't support 32 bit samples.
Merge branch 'libao'
Fix syntax error in call to ao_play()
samples_in is an instance variable, prefix it with ad->
Cast samples_in to samples_out with reinterpret_cast<int16_t*>
Include cstring header for memset()