add "tweakers" functionality

tweakers allow the user to modify construction variables before any build is performed

build_tests/build_dir/tweaker_build.rb

@@ -0,0 +1,12 @@
+Rscons::Environment.new do |env|
+  env.append('CPPPATH' => Dir['src/**/*/'])
+  env.build_dir(%r{^src/([^/]+)/}, 'build_\\1/')
+  env.add_tweaker do |build_op|
+    if build_op[:target] =~ %r{build_one/.*\.o}
+      build_op[:vars]["CFLAGS"] << "-O1"
+    elsif build_op[:target] =~ %r{build_two/.*\.o}
+      build_op[:vars]["CFLAGS"] << "-O2"
+    end
+  end
+  env.Program('tweaker', Dir['src/**/*.c'])
+end