Add 'build' DSL method.

Disallow processing Environments until configuration is performed.

build_tests/two_sources/Rsconscript

@@ -1,4 +1,6 @@
-Rscons::Environment.new(echo: :command) do |env|
-  env.Object("one.o", "one.c", 'CPPFLAGS' => ['-DONE'])
-  env.Program('two_sources.exe', ['one.o', 'two.c'])
+build do
+  Rscons::Environment.new(echo: :command) do |env|
+    env.Object("one.o", "one.c", 'CPPFLAGS' => ['-DONE'])
+    env.Program('two_sources.exe', ['one.o', 'two.c'])
+  end
 end