swfmill: Fix build on GCC 6

https://hydra.nixos.org/build/60900173

pkgs/tools/video/swfmill/default.nix

···
     sha256 = "15mcpql448vvgsbxs7wd0vdk1ln6rdcpnif6i2zjm5l4xng55s7r";
   };
 
+  # Fixes build with GCC 6
+  NIX_CFLAGS_COMPILE = "-std=c++03";
+
   buildInputs = [ pkgconfig libxslt freetype libpng libxml2 ];
 
   meta = {