Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

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.