Add shortcut method for creating environments - close #149

Add env() method to create environments.
Process all environments created at build script top level before
executing any autoconf-enabled tasks, or if no tasks are specified by
the user.

build_tests/clone_env/Rsconscript

@@ -1,12 +1,10 @@
-default do
-  debug = Environment.new(echo: :command) do |env|
-    env['CFLAGS'] = '-O2'
-    env['CPPFLAGS'] = '-DSTRING="Debug Version"'
-    env.Program('program-debug.exe', Dir['src/*.c'])
-  end
-
-  release = debug.clone do |env|
-    env["CPPFLAGS"] = '-DSTRING="Release Version"'
-    env.Program('program-release.exe', Dir['src/*.c'])
-  end
+debug = env(echo: :command) do |env|
+  env['CFLAGS'] = '-O2'
+  env['CPPFLAGS'] = '-DSTRING="Debug Version"'
+  env.Program('program-debug.exe', Dir['src/*.c'])
+end
+
+release = debug.clone do |env|
+  env["CPPFLAGS"] = '-DSTRING="Release Version"'
+  env.Program('program-release.exe', Dir['src/*.c'])
 end