Support for library evolution

[byohay]

Hi,
We've been using JOSESwift for some time, and we want to start precompiling to save build time. I know that this repository doesn't provide binaries, but it would still be helpful for us if this project would be guaranteed to compile with BUILD_LIBRARY_FOR_DISTRIBUTION enabled. Otherwise we might have to diverge from your fork to make it compile.
In practice this probably means creating tests that compile with this flag and check that the build succeeds.
The same was done in GRDB.

@byohay thanks for bringing this up, at the moment there aren't plans to implement this. (For future readers, see #237 (comment) also.)

That being said, we'd welcome a PR if you have to to dig into this for your internal use anyway.

[byohay]

@nathan-mohemian thanks! I opened a PR #267

Glad to hear it! Will take a look tomorrow.