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/library/Rsconscript

@@ -1,6 +1,4 @@
-default do
-  Environment.new(echo: :command) do |env|
-    env.Program('library.exe', ['lib.a', 'three.c'])
-    env.Library("lib.a", ['one.c', 'two.c'], 'CPPFLAGS' => ['-Dmake_lib'])
-  end
+env(echo: :command) do |env|
+  env.Program('library.exe', ['lib.a', 'three.c'])
+  env.Library("lib.a", ['one.c', 'two.c'], 'CPPFLAGS' => ['-Dmake_lib'])
 end

build_tests/library/library_from_object.rb

@@ -1,7 +1,5 @@
-default do
-  Environment.new do |env|
-    env.Program("library.exe", ["lib.a", "three.c"])
-    env.Object("two.o", "two.c")
-    env.Library("lib.a", ["one.c", "two.o"], 'CPPFLAGS' => ['-Dmake_lib'])
-  end
+env do |env|
+  env.Program("library.exe", ["lib.a", "three.c"])
+  env.Object("two.o", "two.c")
+  env.Library("lib.a", ["one.c", "two.o"], 'CPPFLAGS' => ['-Dmake_lib'])
 end

build_tests/library/override_arcmd.rb

@@ -1,6 +1,4 @@
-default do
-  Environment.new(echo: :command) do |env|
-    env["ARCMD"] = %w[ar rcf ${_TARGET} ${_SOURCES}]
-    env.Library("lib.a", glob("*.c"))
-  end
+env(echo: :command) do |env|
+  env["ARCMD"] = %w[ar rcf ${_TARGET} ${_SOURCES}]
+  env.Library("lib.a", glob("*.c"))
 end