Fix build under MacOSX and add buffer_time
This patch fixes two problems I found while trying out the library with OSX and adds one enhancement
Correctly link under darwin (taken from the fink port). This is the same problem as mentioned #727 (closed), I just put it here for completeness.
Fix a lockup if a program is calling ao_close() in an atexit. Coreaudio has its own atexit handler so the HAL has already been taken down by Coreaudio itself. I do not know if this is the best solution but it works.
Add a buffer_time property that allows to set the buffering in the driver. With this property an application can define the buffer_time it wants in ms.