Shipping generated file from generated generator

[Akim Demaille]

Suppose you have a file generated-output that's built from feeding
generator with input.  generator itself is built from
generator.sources.

I would like to ship generated-output (hence it's in srcdir), to spare
the user the need to build it.  I don't know how to do that without
using .SECONDARY, which doesn't appear to be universal amongst makes.
Is there a more portable solution?

Here is a test case, under the form of a... test case :)

interm.test
Description: Binary data

It fails without the .SECONDARY.