Add 'build' DSL method.

Disallow processing Environments until configuration is performed.

build_tests/shared_library/Rsconscript

@@ -1,11 +1,13 @@
-Rscons::Environment.new do |env|
-  env["CPPPATH"] << "src/lib"
-  libmine = env.SharedLibrary("mine", Rscons.glob("src/lib/*.c"))
-  env.Program("test-shared.exe",
-              Rscons.glob("src/*.c"),
-              "LIBPATH" => %w[.],
-              "LIBS" => %w[mine])
-  env.build_after("test-shared.exe", libmine.to_s)
-  env.Program("test-static.exe",
-              Rscons.glob("src/**/*.c"))
+build do
+  Rscons::Environment.new do |env|
+    env["CPPPATH"] << "src/lib"
+    libmine = env.SharedLibrary("mine", Rscons.glob("src/lib/*.c"))
+    env.Program("test-shared.exe",
+                Rscons.glob("src/*.c"),
+                "LIBPATH" => %w[.],
+                "LIBS" => %w[mine])
+    env.build_after("test-shared.exe", libmine.to_s)
+    env.Program("test-static.exe",
+                Rscons.glob("src/**/*.c"))
+  end
 end