wimboot: fix build by fixing -idirafter ordering

`wimboot` build broke after NixOS#210004
where we started dropping default libc include path and switched to
`-idirafter` way of specifying libc headers.

Unfortunately the way it's implemented it injects `-idirafter` after
user's flags, not before. That allows users to inject their paths before
libc include paths, not after (as it would notmally happen).

The change works it around for `wimboot` by pulling `-idirafter` libc
flags before user's flags.

pkgs/tools/misc/wimboot/default.nix

@@ -13,6 +13,15 @@ stdenv.mkDerivation rec {
 
   sourceRoot = "source/src";
 
+  # Workaround '-idirafter' ordering bug in staging-next:
+  #   https://github.com/NixOS/nixpkgs/pull/210004
+  # where libc '-idirafter' gets added after user's idirafter and
+  # breaks.
+  # TODO(trofi): remove it in staging once fixed in cc-wrapper.
+  preConfigure = ''
+    export NIX_CFLAGS_COMPILE_BEFORE_${lib.replaceStrings ["-" "."] ["_" "_"] stdenv.hostPlatform.config}=$(< ${stdenv.cc}/nix-support/libc-cflags)
+  '';
+
   buildInputs = [ libbfd zlib libiberty ];
   makeFlags = [ "wimboot.x86_64.efi" ];