fix sphinx build option assignment in Makefile
fix sphinx build option assignment in Makefile

okuta/chainer,keisuke-umezawa/chainer,okuta/chainer,niboshi/chainer,jnishi/chainer,chainer/chainer,tkerola/chainer,niboshi/chainer,wkentaro/chainer,chainer/chainer,niboshi/chainer,hvy/chainer,ronekko/chainer,chainer/chainer,ktnyt/chainer,ktnyt/chainer,pfnet/chainer,chainer/chainer,wkentaro/chainer,jnishi/chainer,wkentaro/chainer,okuta/chainer,keisuke-umezawa/chainer,hvy/chainer,ktnyt/chainer,jnishi/chainer,hvy/chainer,anaruse/chainer,rezoo/chainer,wkentaro/chainer,ktnyt/chainer,keisuke-umezawa/chainer,niboshi/chainer,hvy/chainer,keisuke-umezawa/chainer,aonotas/chainer,okuta/chainer,jnishi/chainer