oidentd: Set C dialect to gnu89 (broken by GCC 5)

pkgs/servers/identd/oidentd/default.nix

···
 stdenv.mkDerivation rec {
   name = "oidentd-2.0.8";
 
+  CFLAGS = [ "--std=gnu89" ];
+
   src = fetchurl {
     url = "mirror://sourceforge/ojnk/${name}.tar.gz";
     sha256 = "0vzv2086rrxcaavrm3js7aqvyc0grgaqy78x61d8s7r8hz8vwk55";