Disable individual function generation

[g2p]

With the default set of jbuilder warnings, non-public cstructs tend to generate fatal warnings because many functions are generated but not all of them are used.
Instead of annotating the cstruct with an [@@warning "-32"], @avsm suggested generating only a subset of functions.

[avsm]

I think we can extend the annotation syntax to select which set of functions to generate. This is better than generating the unused variable warning I think.