I've had good luck with solvent welding. I think the stuff is mostly MEK, it's sold for welding acrylic but it works on most ABS plastics too. Just be careful not to drip it anywhere as it will melt and scar whatever it gets on. I use a small syringe with a needle to put just a few drips right where I want it. Wicks into the crack and melts the plastic back together.

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I'll second the recommendation for MEK or similar solvent welds. If you don't have a syringe, it can be applied by dipping the eye of a sewing needle into the solvent. Other glues will not prevent the crack from lengthening -- slowly, but it will happen.

A crack may be stopped by drilling a small hole (1mm) at its extent. Obviously such a fix may require some cosmetic work if it is visible.

Whichever fix you choose, practice on spare ABS plastics before trying to fix your Mac.