racket: fix on darwin

pkgs/development/interpreters/racket/default.nix

···
 
   FONTCONFIG_FILE = fontsConf;
   LD_LIBRARY_PATH = libPath;
-  NIX_LDFLAGS = "-lgcc_s";
+  NIX_LDFLAGS = stdenv.lib.optionalString stdenv.cc.isGNU "-lgcc_s";
 
   buildInputs = [ fontconfig libffi libtool makeWrapper sqlite ];
 
···
     cd src/build
   '';
 
-  configureFlags = [ "--enable-shared" "--enable-lt=${libtool}/bin/libtool" ]
-                   ++ stdenv.lib.optional disableDocs [ "--disable-docs" ];
+  shared = if stdenv.isDarwin then "dylib" else "shared";
+  configureFlags = [ "--enable-${shared}" "--enable-lt=${libtool}/bin/libtool" ]
+                   ++ stdenv.lib.optional disableDocs [ "--disable-docs" ]
+                   ++ stdenv.lib.optional stdenv.isDarwin [ "--enable-xonx" ];
 
   configureScript = "../configure";