postgresql_jit: fix darwin build

So, patchelf works on ELF files and these don't exist on Darwin. I'm not
aware of any other way to eliminate all references of the executable's
body while preserving library paths in the header. This still works, the
only problem is that darwin has a dependency on llvm.dev, so it's
runtime closure is larger.

pkgs/servers/sql/postgresql/default.nix

@@ -164,11 +164,13 @@ let
             --replace ${self.llvmPackages.llvm.dev}/bin/llvm-config llvm-config \
             --replace -I${self.llvmPackages.llvm.dev}/include ""
 
-          # Stop lib depending on the -dev output of llvm
-          rpath=$(patchelf --print-rpath $out/lib/llvmjit.so)
-          nuke-refs -e $out $out/lib/llvmjit.so
-          # Restore the correct rpath
-          patchelf $out/lib/llvmjit.so --set-rpath "$rpath"
+          ${lib.optionalString (!stdenv'.isDarwin) ''
+            # Stop lib depending on the -dev output of llvm
+            rpath=$(patchelf --print-rpath $out/lib/llvmjit.so)
+            nuke-refs -e $out $out/lib/llvmjit.so
+            # Restore the correct rpath
+            patchelf $out/lib/llvmjit.so --set-rpath "$rpath"
+          ''}
         ''}
       '';