So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

Since `this` was captured, it does not need to be specified, but GCC 6
can't do it.  Since we want to support the standard compiler from
Debian Stretch (stable), we need to work around this problem:

 src/neighbor/plugins/UdisksNeighborPlugin.cxx:239:12: error: cannot call member function 'void UdisksNeighborExplorer::Insert(UDisks2::Object&&)' without object
       Insert(std::move(o));
       ~~~~~~^~~~~~~~~~~~~~

This fixes #300 which I previously thought was a different bug.

Fixes assertion failure.

Allows other objects to use the same single instance as well.

To get udisks2 support started, this commit contains the configure.ac
option and a "neighbor" plugin which shows block devices.  Later, this
will allow mounting removable media with a new storage plugin.

We can do CURL requests asynchronously, and we don't need a
synchronous WorkQueue thread for that.

This allows parallelizing lookups and allows immediate cancellation.

We can remove those C++11 and C++14 kludges because we require GCC 4.9
now.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

The old struct config_param remains only for top-level string options.