Add 'build' DSL method.

Disallow processing Environments until configuration is performed.

lib/rscons/script.rb

@@ -19,6 +19,11 @@ module Rscons
         @script.autoconf = autoconf
       end
 
+      # Enter build block.
+      def build(&block)
+        @script.operations["build"] = block
+      end
+
       # Enter configuration block.
       def configure(&block)
         @script.operations["configure"] = block
@@ -81,6 +86,13 @@ module Rscons
       Dsl.new(self).instance_eval(script_contents, path, 1)
     end
 
+    # Perform build operation.
+    def build
+      if build_proc = @operations["build"]
+        build_proc.call
+      end
+    end
+
     # Perform configure operation.
     def configure(configure_op)
       if operation_lambda = @operations["configure"]