wip for precompile phase for D

build_tests/d/Rsconscript

@@ -3,5 +3,6 @@ configure do
 end
 
 env(echo: :command) do |env|
-  env.Program("hello-d.exe", glob("*.d"))
+  env["D_IMPORT_PATH"] << "src"
+  env.Program("hello-d.exe", glob("src/*.d"))
 end