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Now the resources are installed and a lookup is performed to find them from
either (respectively) an environment var, the user's datadir and the
compile-time datadir.
Also the server is responsible for sharing the xml data so that all clients
get the same thing, from him over network.
Also libovcc is responsible for the DTD files (since it's a sort of spec)
while the actual XML data belongs to the server.

Workaround the load of OVCCGtkMessageInfo as type-func seems currently
sort-of broken (unused), by pre-loading the type forcefully.
See: https://bugzilla.gnome.org/show_bug.cgi?id=786932

This makes OVCCGtkBoard a more normal widget by allowing to change the
OVCCBoard it displays, avoiding the need to create and destroy the
widget.

This gives better consistency by having tile and pawn related signals
next to each other, as well as making the board more self-sufficient
with it getting signals for all changes on it.

Now we don't install other data next to the link, we must make sure the
directory is created.

Make variable "$<" in non-suffix rules is a GNU Make extension, so
switch to suffix rules.

Additionally, make the rules a little more generic by extracting custom
flags to a _FLAGS variable.

Also drop the build-time support for vertical layout, it's useless,
less nice, adds complexity and needs different packing options.

Add an icon for the application, based on the tile 13 by Salomé.

We use libgthread, but we didn't explicitly link it.  This used to
work because other libraries we link also links libgthread, but such
indirect dependencies are rejected by modern linkers.