racket-minimal: build on darwin

Technically, racket compiles and packages too, but it doesn't pass all
tests yet, so the build fails.

gcc7Stdenv is GCC 7 with the MacOS-specific adaptations retained.
Building on MacOS with that works, but we should be using clang.

I'm hoping that the next person wanting to override the compiler
version in some other derivation sees that attribute and doesn't
inadvertently break a MacOS build again.