Add serf support via scons

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
subversion 1.6.23

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
subversion 1.7.14

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>