add Environment#clone() and a build test for it

build_tests/clone_env/build.rb

@@ -0,0 +1,11 @@
+debug = Rscons::Environment.new do |env|
+  env.build_dir('src', 'debug')
+  env['CFLAGS'] = '-O2'
+  env['CPPFLAGS'] = '-DSTRING="Debug Version"'
+  env.Program('program-debug', Dir['src/*.c'])
+end
+
+release = debug.clone('CPPFLAGS' => '-DSTRING="Release Version"') do |env|
+  env.build_dir('src', 'release')
+  env.Program('program-release', Dir['src/*.c'])
+end

build_tests/clone_env/src/program.c

@@ -0,0 +1,6 @@
+#include <stdio.h>
+
+int main(int argc, char *argv[])
+{
+    printf("Hello, %s\n", STRING);
+}