Lets make mmio/mmap alsa support configurable at run time.

The option is "use_mmap".  Accepted values for enabling are:
"yes","y","true","t","1".  Anything else disables mmap support.
If mmio is enable at compile time, use that as the default.


git-svn-id: http://svn.xiph.org/trunk/ao@5456 0101bb08-14d6-0310-b084-bc0e0c8e3800