Add 'build' DSL method.

Disallow processing Environments until configuration is performed.

build_tests/custom_builder/Rsconscript

@@ -9,8 +9,10 @@ EOF
   end
 end
 
-Rscons::Environment.new do |env|
-  env.add_builder(MySource.new)
-  env.MySource('inc.h', [])
-  env.Program('program.exe', Dir['*.c'])
+build do
+  Rscons::Environment.new do |env|
+    env.add_builder(MySource.new)
+    env.MySource('inc.h', [])
+    env.Program('program.exe', Dir['*.c'])
+  end
 end