The custom command that links conflict-set.o depends on
$<TARGET_OBJECTS:conflict-set-object>, but that generator expression
does not create a target-level dependency. With the Unix Makefiles
generator, parallel builds can start building the consuming libraries
before the object library's build rule is available, producing:
gmake[3]: *** No rule to make target
'CMakeFiles/conflict-set-object.dir/ConflictSet.cpp.o', needed by
'conflict-set.o'. Stop.
Add add_dependencies() so that conflict-set and conflict-set-static
cannot build until conflict-set-object has produced its object files.
Closes#47
