Refactor mkFlag / shouldUsePkg into the nixpkgs libraries

pkgs/development/libraries/wiredtiger/default.nix

@@ -5,17 +5,8 @@
 , gperftools ? null, leveldb ? null
 }:
 
-with stdenv.lib;
+with stdenv;
 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;
@@ -24,6 +15,7 @@ let
   optGperftools = shouldUsePkg gperftools;
   optLeveldb = shouldUsePkg leveldb;
 in
+with stdenv.lib;
 stdenv.mkDerivation rec {
   name = "wiredtiger-${version}";
   version = "2.6.0";