Hey, are you still working on this? If so, you could try adding glibc.dev to your targetPkgs, or try adding "dev" to extraOutputsToInstall.
I’m currently also trying to use a buildFHSUserEnv, but I’d like it to work with gcc-unwrapped. If you have time I’d be super happy if you could take a look: Using a "raw" `gcc` inside `buildFHSUserEnv`. Thanks anyway!