thunderbird: Fix build on gcc 5

It was barfing due to --enable-stdcxx-compat. Not clear if/why we
still need this, so let's disable it. If necessary a fix is available
at https://bugzilla.mozilla.org/show_bug.cgi?id=1153109.

pkgs/applications/networking/mailreaders/thunderbird/default.nix

@@ -69,7 +69,7 @@ stdenv.mkDerivation rec {
                                "--enable-optimize" "--enable-strip" ])
     ++ [
       "--disable-javaxpcom"
-      "--enable-stdcxx-compat" # Avoid dependency on libstdc++ 4.7
+      #"--enable-stdcxx-compat" # Avoid dependency on libstdc++ 4.7
     ]
     ++ stdenv.lib.optional enableOfficialBranding "--enable-official-branding";
   in ''