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/simple_cc/Rsconscript

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

build_tests/simple_cc/cxxsuffix.rb

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

build_tests/simple_cc/link_objects.rb

@@ -1,6 +1,4 @@
-default do
-  Environment.new do |env|
-    env.Object("simple.o", "simple.cc")
-    env.Program("simple.exe", "simple.o")
-  end
+env do |env|
+  env.Object("simple.o", "simple.cc")
+  env.Program("simple.exe", "simple.o")
 end

build_tests/simple_cc/preprocess.rb

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