Sometimes you want to optionally use a new feature of libdiscid without having to depend on a recent libdiscid.

An example is LIB-29. Users would want to use read_sparse() when available but just keep using read() when compiling against libdiscid <= 0.2.2.

We didn't introduce any defines for the ISRC API, but we did introduce something for the (disc read) features in LIB-23.

If this is about using new functions we need defines.
These can technically be generated by the build files of the user, our build files or in the includes (directly).

We can use version number defines (major, minor, patch or api number) or defines for special features.