add gapbuffer

Rsconscript

@@ -3,9 +3,13 @@ configure do
 end
 
 build do
-  Rscons::Environment.new do |env|
+  main_env = Rscons::Environment.new do |env|
     env["sources"] = glob("src/**/*.{c,d}")
     env["D_IMPORT_PATH"] += glob("src/**")
     env.Program("jes", "${sources}")
   end
+  main_env.clone do |env|
+    env["DFLAGS"] += %w[-g -funittest]
+    env.Program("jestest", "${sources}")
+  end
 end