Fix up definition usage

[erincatto]

erincatto/box2d#762

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Just an FYI that on the Mac, I get errors compiling the samples. I can work around just by not building the samples.

/box2c/samples/sample_shapes.cpp:304:36: error: taking the address of a temporary object of type 'b2ShapeDef' [-Waddress-of-temporary]
b2CreatePolygonShape(m_ship1Id, &b2DefaultShapeDef(), &right);
^~~~~~~~~~~~~~~~~~~~
...
^~~~~~~~~~~~~~~~~~~
/box2c/samples/sample_shapes.cpp:473:35: error: taking the address of a temporary object of type 'b2ShapeDef' [-Waddress-of-temporary]
b2CreateSegmentShape(groundId, &b2DefaultShapeDef(), &segment);
[ X] Ask for help on discord and/or reddit
[ X] Consider providing a dump file using b2World::Dump

[erincatto]

Try adding samples to macos CI

[erincatto]

Added samples to macOS CI in #130