Refactor mkFlag / shouldUsePkg into the nixpkgs libraries

pkgs/development/libraries/libxml2/default.nix

@@ -6,16 +6,8 @@
 
 #TODO: share most stuff between python and non-python builds, perhaps via multiple-output
 
+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 && stdenv.lib.any (x: x == stdenv.system) pkg.meta.platforms then pkg else null;
-
   optIcu = shouldUsePkg icu;
   optPython = shouldUsePkg python;
   optReadline = shouldUsePkg readline;
@@ -25,6 +17,7 @@ let
   sitePackages = if optPython == null then null else
     "\${out}/lib/${python.libPrefix}/site-packages";
 in
+with stdenv.lib;
 stdenv.mkDerivation rec {
   name = "libxml2-${version}";
   version = "2.9.2";