this code generates "cannot coerce a function to a string" error but I don't get why ```
postBuild = with stdenv.lib; builtins.toString(builtins.concatStringsSep "\\" [ optionalString generateBindings "./waf --apiscan=all "
       stdenv.lib.optionalString withDoc "./waf doxygen"
       stdenv.lib.optionalString withManual "./waf sphinx"
      ])
    ;```