Mac build was failing due to undefined "executable"

Take the executable declaration code from upstream

indra/newview/viewer_manifest.py

@@ -1535,6 +1535,7 @@ class Darwin_x86_64_Manifest(ViewerManifest):
                         self.path(libfile)
 
                         oldpath = os.path.join("@rpath", libfile)
+                        executable = self.dst_path_of(self.channel())
                         self.run_command(
                             ['install_name_tool', '-change', oldpath,
                              '@executable_path/../Resources/%s' % libfile,