Mark some packages as broken due to GCC 5

pkgs/development/libraries/openbabel/default.nix

···
   meta = {
     platforms = stdenv.lib.platforms.all;
     maintainers = [ stdenv.lib.maintainers.urkud ];
+    broken = true; # doesn't build with GCC 5; fix in GitHub
   };
 }

pkgs/development/libraries/vxl/default.nix

···
     license = "VXL License";
     maintainers = with stdenv.lib.maintainers; [viric];
     platforms = with stdenv.lib.platforms; linux;
+    broken = true; # requires fix for gcc 5: https://lists.fedoraproject.org/pipermail/scm-commits/Week-of-Mon-20150216/1511118.html
   };
 }