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/cfile/Rsconscript

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

build_tests/cfile/error_unknown_extension.rb

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