Pre-build hooks do not respect modified key values for old-style builders

build_tests/build_dir/backward_compatible_build_hooks.rb

@@ -0,0 +1,22 @@
+class MyObject < Rscons::Builder
+  def run(target, sources, cache, env, vars)
+    env.run_builder(env.builders["Object"], target, sources, cache, vars)
+  end
+end
+
+Rscons::Environment.new(echo: :command) do |env|
+  env.add_builder(MyObject.new)
+  env.append('CPPPATH' => Dir['src/**/*/'].sort)
+  env.add_build_hook do |build_op|
+    if build_op[:builder].name == "MyObject" && build_op[:sources].first =~ %r{one\.c}
+      build_op[:vars]["CFLAGS"] << "-O1"
+      build_op[:sources] = ['src/two/two.c']
+    elsif build_op[:builder].name == "MyObject" && build_op[:target] =~ %r{two\.o}
+      new_vars = build_op[:vars].clone
+      new_vars["CFLAGS"] << "-O2"
+      build_op[:vars] = new_vars
+    end
+  end
+  env.MyObject('one.o', 'src/one/one.c')
+  env.MyObject('two.o', 'src/two/two.c')
+end