[PROJ] Motion: Adopt RFC3 - Dependency management

[Greg Troxel]

Kristian Evers <kreve at sdfe.dk> writes:

> Yes, I agree that we should stay on the conservative side with regards
> to the API. How about we set up tests on travis and appveyor that
> checks that the API can be used in a strictly C89 application? Perhaps
> also a similar thing for the C++ API.

I think it's very important that the library can continue to be used
from C.  But I wonder how much C89 matters, vs C99?   In headers,
though, I'm not sure how much more C89 hurts.