This includes the textual version number in its existing
form (yyyymmdd followed by an abbreviated git hash). The four-part
binary version is set to 1 followed by year, month and day; if I ever
want to change that, I can increment the initial 1.
FileDescription is taken from the existing DESCRIPTION string provided
to each puzzle() statement in CMakeLists.txt.
This means that puzzles.rc now always defines at least one resource,
so we can remove the workaround for MinGW's windres not being able to
cope with an empty .rc file, which added a dummy resource in the
absence of an icon.
