Versions

AY2D_ABI_VERSION (in the header) and ay2d_abi_version() (in the library) must match. Check once at startup.
The ABI version changes only when an existing function, struct or behaviour changes. New functions, flags, statuses, or fields added to the end of a struct do not change it, so a program built against an older header keeps working with a newer library.
The header is generated from the library's source, so it never describes something the library does not do.
ay2d_version() is the Ayatsuri2D release the library came from.