Always use a build root and default it to "build"

build_tests/custom_builder/build_after.rb

@@ -3,6 +3,6 @@ Rscons::Environment.new do |env|
               [],
               "CMD" => %w[ruby gen.rb ${_TARGET}],
               "CMD_DESC" => "Generating")
-  env.build_after("program.o", "inc.c")
+  env.build_after("#{env.build_root}/program.o", "inc.c")
   env.Program("program.exe", ["program.c", "inc.c"])
 end