stdenv/adapters.nix: fixup makeStaticBinaries

- makeStaticBinaries don’t work on Darwin (no stable ABI!)
- Need to make sure NIX_CFLAGS_LINK appends
- isStatic is not used anymore

pkgs/stdenv/adapters.nix

@@ -32,13 +32,15 @@ rec {
   # Return a modified stdenv that tries to build statically linked
   # binaries.
   makeStaticBinaries = stdenv: stdenv //
-    { mkDerivation = args: stdenv.mkDerivation (args // {
-        NIX_CFLAGS_LINK = "-static";
+    { mkDerivation = args:
+      if stdenv.hostPlatform.isDarwin
+      then throw "Cannot build fully static binaries on Darwin/macOS"
+      else stdenv.mkDerivation (args // {
+        NIX_CFLAGS_LINK = toString (args.NIX_CFLAGS_LINK or "") + "-static";
         configureFlags = (args.configureFlags or []) ++ [
             "--disable-shared" # brrr...
           ];
       });
-      isStatic = true;
     };