Add function prototypes to interface

[ryandesign]

At the point where distanceBetweenPoint or cubicEaseInOut are invoked, old compilers complain:

error: incompatible types in initialization

This is solved by declaring the function prototypes in the interface.

[BlueM]

The methods are deliberately not in the header file, as they aren’t meant to be regarded as public methods. Which old compilers (on which system versions) complain? I’m asking to verify which workaround to choose.