Old order in Makefile was problematic.

Makefile:

$(CC) $(CFLAGS) -o $(EXEC) grid.o state.o king.o network.o output.o client.o server.o main.o $(LDFLAGS)

The order is problematic. Better use

$(CC) $(CFLAGS) $(LDFLAGS) grid.o state.o king.o network.o output.o client.o server.o main.o -o $(EXEC)

to avoid a FTBFS with binutils-gold.

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Build and execution is tested: all ok.