Refactor mkFlag / shouldUsePkg into the nixpkgs libraries

pkgs/development/libraries/nghttp2/default.nix

@@ -8,16 +8,9 @@
 , prefix ? ""
 }:
 
+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 && stdenv.lib.any (x: x == stdenv.system) pkg.meta.platforms then pkg else null;
-
   isLib = prefix == "lib";
 
   optOpenssl = if isLib then null else shouldUsePkg openssl;
@@ -50,7 +43,7 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ pkgconfig ];
   buildInputs = [ optJansson optBoost optLibxml2 optJemalloc ]
-    ++ stdenv.lib.optionals hasApp [ optOpenssl optLibev optZlib ];
+    ++ optionals hasApp [ optOpenssl optLibev optZlib ];
 
   configureFlags = [
     (mkEnable false                 "werror"          null)
@@ -68,7 +61,7 @@ stdenv.mkDerivation rec {
     (mkWith   false                 "cython"          null)
   ];
 
-  meta = with stdenv.lib; {
+  meta = {
     homepage = http://nghttp2.org/;
     description = "an implementation of HTTP/2 in C";
     license = licenses.mit;