Add 'build' DSL method.

Disallow processing Environments until configuration is performed.

build_tests/library/Rsconscript

@@ -1,4 +1,6 @@
-Rscons::Environment.new(echo: :command) do |env|
-  env.Program('library.exe', ['lib.a', 'three.c'])
-  env.Library("lib.a", ['one.c', 'two.c'], 'CPPFLAGS' => ['-Dmake_lib'])
+build do
+  Rscons::Environment.new(echo: :command) do |env|
+    env.Program('library.exe', ['lib.a', 'three.c'])
+    env.Library("lib.a", ['one.c', 'two.c'], 'CPPFLAGS' => ['-Dmake_lib'])
+  end
 end