Refactor mkFlag / shouldUsePkg into the nixpkgs libraries

pkgs/development/libraries/libmicrohttpd/default.nix

@@ -5,16 +5,8 @@
 , openssl ? null, zlib ? null, libgcrypt ? null, gnutls ? null
 }:
 
+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;
-
   optOpenssl = shouldUsePkg openssl;
   optZlib = shouldUsePkg zlib;
   hasSpdy = optOpenssl != null && optZlib != null;