Refactor mkFlag / shouldUsePkg into the nixpkgs libraries

pkgs/development/libraries/libgcrypt/default.nix

@@ -5,16 +5,9 @@
 , libcap ? null, pth ? null
 }:
 
+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;
-
   optLibcap = shouldUsePkg libcap;
   #optPth = shouldUsePkg pth;
   optPth = null; # Broken as of 1.6.3
@@ -38,13 +31,13 @@ stdenv.mkDerivation rec {
   # Also make sure includes are fixed for callers who don't use libgpgcrypt-config
   postInstall = ''
     sed -i 's,#include <gpg-error.h>,#include "${libgpgerror}/include/gpg-error.h",g' $out/include/gcrypt.h
-  '' + stdenv.lib.optionalString (!stdenv.isDarwin && optLibcap != null) ''
+  '' + optionalString (!stdenv.isDarwin && optLibcap != null) ''
     sed -i 's,\(-lcap\),-L${optLibcap}/lib \1,' $out/lib/libgcrypt.la
   '';
 
   doCheck = true;
 
-  meta = with stdenv.lib; {
+  meta = {
     homepage = https://www.gnu.org/software/libgcrypt/;
     description = "General-pupose cryptographic library";
     license = licenses.lgpl2Plus;