Add 'build' DSL method.

Disallow processing Environments until configuration is performed.

build_tests/cfile/Rsconscript

@@ -1,4 +1,6 @@
-Rscons::Environment.new do |env|
-  env.CFile("lexer.c", "lexer.l")
-  env.CFile("parser.c", "parser.y")
+build do
+  Rscons::Environment.new do |env|
+    env.CFile("lexer.c", "lexer.l")
+    env.CFile("parser.c", "parser.y")
+  end
 end

build_tests/cfile/error_unknown_extension.rb

@@ -1,3 +1,5 @@
-Rscons::Environment.new do |env|
-  env.CFile("file.c", "foo.bar")
+build do
+  Rscons::Environment.new do |env|
+    env.CFile("file.c", "foo.bar")
+  end
 end