A C-based alternative for compiling ZIL? I don't believe there's one that works. The two options for compiling ZIL code are to run ZILF on a modern system or to run ZILCH on a PDP-10 emulator.
The good news is you don't need to install any C# development tools if you use the prepackaged binaries, since they're standalone executables.
https://notabug.org/coderain/zilutils
EDIT: Wrong link. Wait, I'm sure there's a C based alternative out there.