Make Tiny C89 compliant

[msteinbeck]

Hi @andsmedeiros,

thank you for creating and sharing this library. I would like to use it in my project (https://github.com/msteinbeck/tinyspline). I'm planning to replace explicit calls to malloc and free with preprocessor definitions, allowing users of TinySpline to define them with custom implementations (e.g., if memory must be allocated on stack). In order to test these definitions, I need a smell, yet functional implementation of malloc and free for my unit tests.

Tiny is implemented in c11. Because TinySpline is implemented in c89, I cannot make use of your code. Can I convince you to make your code c89 compliant?