CMake: template_debug as default target when godot-cpp is top level #1728
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People seem to want to build godot-cpp by itself, without any arguments, not even specifying a target.
So we have to detect if we are top level and make template_debug available for the all target.
This can only happen when we are the top level target, because if we enable template_debug for all when we are part of something larger than its unnecessary compilation.
This adresses #1724 and duplicate #1727