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.

Doesn't work yet, just to get the Android port started.

This is what we get from the storage plugin via FileInfo.  Fixes a
compiler warning on Mac OS X where dev_t appears to be signed.

Define "ANDROID" if the host operating system is Android.

Fixes build failure caused by commit d9466cad.

Fixes nullptr dereference (regression by commit df5f9f4a).

Prepare for providing everything in a shared library, to be embedded
in another process.

Ignore the storage configuration if FLAG_REQUIRE_STORAGE is not set in
the DatabasePlugin.

The new method IsPlugin() replaces the "is_simple" flag.

Use a listener interface instead of GlobalEvents.

Build was broken by previous commit.