[Jigsy]

I'm guessing this no longer qualifies as "modern," since the last update was in 2018 and is no longer in active development, but I'd like to say that the 32bit version of the Tiny C Compiler by Fabrice Bellard works on Windows 98 SE.

https://www.bellard.org/tcc/

[thegeekpirate]

There's a fork which has been continually worked on https://repo.or.cz/w/tinycc.git

Unofficial mirror https://github.com/TinyCC/tinycc