Add 'build' DSL method.

Disallow processing Environments until configuration is performed.

build_tests/simple/register_target_in_build_hook.rb

@@ -1,8 +1,10 @@
-Rscons::Environment.new do |env|
-  env.Program("simple.exe", Dir["*.c"])
-  env.add_build_hook do |build_op|
-    if build_op[:target].end_with?(".o")
-      env.Disassemble("#{build_op[:target]}.txt", build_op[:target])
+build do
+  Rscons::Environment.new do |env|
+    env.Program("simple.exe", Dir["*.c"])
+    env.add_build_hook do |build_op|
+      if build_op[:target].end_with?(".o")
+        env.Disassemble("#{build_op[:target]}.txt", build_op[:target])
+      end
     end
   end
 end