Add 'build' DSL method.

Disallow processing Environments until configuration is performed.

build_tests/simple_cc/Rsconscript

@@ -1,3 +1,5 @@
-Rscons::Environment.new do |env|
-  env.Program('simple.exe', Dir['*.cc'])
+build do
+  Rscons::Environment.new do |env|
+    env.Program('simple.exe', Dir['*.cc'])
+  end
 end

build_tests/simple_cc/cxxsuffix.rb

@@ -1,5 +1,7 @@
-Rscons::Environment.new do |env|
-  env["CXXSUFFIX"] = %w[.cccc .cc]
-  env["CXXFLAGS"] += %w[-x c++]
-  env.Program("simple.exe", Dir["*.cc"] + Dir["*.cccc"])
+build do
+  Rscons::Environment.new do |env|
+    env["CXXSUFFIX"] = %w[.cccc .cc]
+    env["CXXFLAGS"] += %w[-x c++]
+    env.Program("simple.exe", Dir["*.cc"] + Dir["*.cccc"])
+  end
 end

build_tests/simple_cc/preprocess.rb

@@ -1,4 +1,6 @@
-Rscons::Environment.new do |env|
-  env.Preprocess("simplepp.cc", "simple.cc")
-  env.Program("simple.exe", "simplepp.cc")
+build do
+  Rscons::Environment.new do |env|
+    env.Preprocess("simplepp.cc", "simple.cc")
+    env.Program("simple.exe", "simplepp.cc")
+  end
 end