Revert "Refactor mkFlag / shouldUsePkg into the nixpkgs libraries"

This reverts commit 25a148fa19.

pkgs/development/libraries/wiredtiger/default.nix

@@ -5,8 +5,17 @@
 , gperftools ? null, leveldb ? null
 }:
 
-with stdenv;
+with stdenv.lib;
 let
+  mkFlag = trueStr: falseStr: cond: name: val:
+    if cond == null then null else
+      "--${if cond != false then trueStr else falseStr}${name}${if val != null && cond != false then "=${val}" else ""}";
+  mkEnable = mkFlag "enable-" "disable-";
+  mkWith = mkFlag "with-" "without-";
+  mkOther = mkFlag "" "" true;
+
+  shouldUsePkg = pkg: if pkg != null && any (x: x == stdenv.system) pkg.meta.platforms then pkg else null;
+
   optLz4 = shouldUsePkg lz4;
   optSnappy = shouldUsePkg snappy;
   optZlib = shouldUsePkg zlib;
@@ -15,7 +24,6 @@ let
   optGperftools = shouldUsePkg gperftools;
   optLeveldb = shouldUsePkg leveldb;
 in
-with stdenv.lib;
 stdenv.mkDerivation rec {
   name = "wiredtiger-${version}";
   version = "2.6.0";