Refactor mkFlag / shouldUsePkg into the nixpkgs libraries

pkgs/development/libraries/kerberos/heimdal.nix

@@ -8,16 +8,9 @@
 #, sqlite, db, ncurses, openssl, cyrus_sasl
 }:
 
+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;
-
   optOpenldap = shouldUsePkg openldap;
   optLibcap_ng = shouldUsePkg libcap_ng;
   optSqlite = shouldUsePkg sqlite;
@@ -97,7 +90,7 @@ stdenv.mkDerivation rec {
     rmdir $out/libexec
   '';
 
-  meta = with stdenv.lib; {
+  meta = {
     description = "an implementation of Kerberos 5 (and some more stuff) largely written in Sweden";
     license = licenses.bsd3;
     platforms = platforms.linux;