exempi: fix build on darwin

pkgs/top-level/all-packages.nix

@@ -5655,8 +5655,10 @@ with pkgs;
   };
 
   exempi = callPackage ../development/libraries/exempi {
-    stdenv = if stdenv.isi686 then gcc6Stdenv else gcc9Stdenv;
-    boost  = boost15x;
+    stdenv = if stdenv.isDarwin then stdenv
+             else if stdenv.isi686 then gcc6Stdenv
+             else gcc9Stdenv;
+    boost  = if stdenv.isDarwin then boost else boost15x;
   };
 
   execline = skawarePackages.execline;