Hi!

Ubuntu 24.04 LTS is what many people use and what GitHub Actions will continue to offer for most recent Ubuntu until 26.04 comes along. As of today, Ubuntu 24.04 has:

• Protobuf 3.21.12 — https://packages.ubuntu.com/source/noble/protobuf
• Abseil 20220623.1 — https://packages.ubuntu.com/source/noble/abseil

When trying to integrate libprotobuf-mutator into a Ubuntu 24.04 based build pipeline at libexpat/libexpat#950 I had to resort to libprotobuf-mutator as old as v1.1-7-g50ef915 (commit 50ef915) because nothing more recent would compile and link with these versions of Protobuf and Abseil from Ubuntu 24.04.

I'm aware that you have LIB_PROTO_MUTATOR_DOWNLOAD_PROTOBUF here to bypass host Protobuf and Abseil but I have seen it take 20+ minutes to compile and that's not something can do for every run of GitHub Actions, even less so in a private repository than a public one because of CI minute limits and cost.

I have a feeling it could be possible to restore support for these versions of Protobuf and Abseil by adding some #ifdefs related to the version of Protobuf and Abseil being compiled against. Something like that would make libprotobuf-mutator a lot more friendly to use by the ecosystem.

Thanks for your consideration 🙏

Best, Sebastian