add meta.branch

pkgs/development/libraries/fox/fox-1.6.nix

···
   enableParallelBuilding = true;
 
   meta = {
+    branch = "1.6";
     description = "FOX is a C++ based class library for building Graphical User Interfaces";
     longDescription = ''
         FOX stands for Free Objects for X.

pkgs/development/libraries/gmp/4.3.2.nix

···
   doCheck = false;
 
   meta = {
+    branch = "4";
     description = "GNU multiple precision arithmetic library";
 
     longDescription =

pkgs/development/tools/parsing/flex/2.5.35.nix

···
   };
 
   meta = {
+    branch = "2.5.35";
     homepage = http://flex.sourceforge.net/;
     description = "A fast lexical analyser generator";
   };