That definitely ups the stakes on the "modern" approach a lot (like, in the limit case `#ifdef`-hell to get part way there).
There are firms that will sell you a suite of frontends supporting every C compiler back to the early 80s and integrate them into a modern toolchain, I used to work with an alum of such a firm and I gather it's great stuff. I also gather it costs whatever you can afford, so there's that. I forget the name of the company but I could ping my friend if that's interesting to you.
Worst case, you could not have to chase memory corruptions on the subset of your target platforms that LLVM targets.
There are firms that will sell you a suite of frontends supporting every C compiler back to the early 80s and integrate them into a modern toolchain, I used to work with an alum of such a firm and I gather it's great stuff. I also gather it costs whatever you can afford, so there's that. I forget the name of the company but I could ping my friend if that's interesting to you.
Worst case, you could not have to chase memory corruptions on the subset of your target platforms that LLVM targets.