Gnuchess snap.

To build:

 1. make snap

This will:

 1. Fetch gnuchess, xboard and the book's png files.
 2. Build the book.bin, for which it needs to build gnuchess itself.
 3. Create a snap with gnuchess, xboard, and the book.bin

Notes:

Xboard integration is not fully working yet. It needs --devmode, and I prefer
not to release a devmode snap, as it's a crutch to get over snap limitations
which should not affect xboard. I'll wait until snap grows support for the
proper interface.