rpPPPoE: support cross compile

- prevent a configure time check from running. provided an answer that
  works for x86_64-linux and aarch64-linux
- point the makefile to the correct ar tool

pkgs/tools/networking/rp-pppoe/default.nix

@@ -16,11 +16,16 @@ stdenv.mkDerivation rec {
     cd src
     export PPPD=${ppp}/sbin/pppd
   '';
+
+  configureFlags = lib.optionals (stdenv.buildPlatform != stdenv.hostPlatform) [ "rpppoe_cv_pack_bitfields=rev" ];
+
   postConfigure = ''
     sed -i Makefile -e 's@DESTDIR)/etc/ppp@out)/etc/ppp@'
     sed -i Makefile -e 's@PPPOESERVER_PPPD_OPTIONS=@&$(out)@'
   '';
 
+  makeFlags = [ "AR:=$(AR)" ];
+
   meta = with lib; {
     description = "Roaring Penguin Point-to-Point over Ethernet tool";
     platforms = platforms.linux;