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.