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.

pkgs/development/interpreters/racket/default.nix

···
 , libpng, libtool, mpfr, openssl, pango, poppler
 , readline, sqlite
 , disableDocs ? false
+, CoreFoundation
 }:
 
 let
···
 
   FONTCONFIG_FILE = fontsConf;
   LD_LIBRARY_PATH = libPath;
-  NIX_LDFLAGS = stdenv.lib.optionalString stdenv.cc.isGNU "-lgcc_s";
+  NIX_LDFLAGS = stdenv.lib.concatStringsSep " " [
+    (stdenv.lib.optionalString (stdenv.cc.isGNU && ! stdenv.isDarwin) "-lgcc_s")
+    (stdenv.lib.optionalString stdenv.isDarwin "-framework CoreFoundation")
+  ];
 
-  buildInputs = [ fontconfig libffi libtool makeWrapper sqlite ];
+  buildInputs = [ fontconfig libffi libtool makeWrapper sqlite ]
+    ++ stdenv.lib.optionals stdenv.isDarwin [ CoreFoundation ];
 
   preConfigure = ''
     unset AR

pkgs/development/interpreters/racket/minimal.nix

···
       as well as libraries that live in collections. In particular, raco
       and the pkg library are still bundled.
     '';
-    platforms = [ "x86_64-linux" "aarch64-linux" ];
+    platforms = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" ];
   };
 })

pkgs/top-level/all-packages.nix

···
     extraBuildInputs = lib.optional hostPlatform.isDarwin clang.cc;
   };
 
+  gcc7Stdenv = overrideCC gccStdenv gcc7;
+
   wrapCCMulti = cc:
     if system == "x86_64-linux" then let
       # Binutils with glibc multi
···
     # racket 6.11 doesn't build with gcc6 + recent glibc:
     # https://github.com/racket/racket/pull/1886
     # https://github.com/NixOS/nixpkgs/pull/31017#issuecomment-343574769
-    stdenv = overrideCC stdenv gcc7;
+    stdenv = if stdenv.isDarwin then stdenv else gcc7Stdenv;
+    inherit (darwin.apple_sdk.frameworks) CoreFoundation;
   };
   racket-minimal = callPackage ../development/interpreters/racket/minimal.nix { };