I tried to compile an old Win32 C program with a build.zig and didn't see an obvious happy path to compile an .rc file into a .res file and add it to the build. These seem to be somewhat common in Win32 projects, so I think that if the intention is to support gradually converting Win32 C programs to Zig, being able to easily compile resource files and add them to the target should be part of that.

LLVM comes with llvm-rc, and it worked for me, but unlike windres and whatever the MSVC resource compiler is, llvm-rc doesn't run the preprocessor against the source file, so a preprocessing step with clang -E is also necessary. However, once you get a .res file out of it, it can be added to the build with addObjectFile.