Break the mainloop and terminate run call.

Let the user know that MPD has failed.  Not the best thing to do, but
better than pretending it still runs.

Add class Context which wraps the Android/Java Context class and add a
JNI wrapper for method Context.getCacheDir().

We're not using external storage yet, but as soon as we will, we only
need to read music files, not write them.

Allow library auto-detection without pkg-config.

Finally, MPD runs on Android.  For some small value of "runs".  Very
much work left, too much to describe.