给Ubuntu系统安装VirtualBox Guest Additions，如果出现错误：
Recompiling VirtualBox kernel module [FAILED]
(Look at /var/log/vbox-install.log to find out what went wrong)"
查看log显示：
Makefile:75: *** Error: unable to find the sources of yourcurrent Linux kernel. Specify KERN_DIR= and run Make again.
解决方法为安装linux header，运行下面的命令，然后重新安装即可：
sudo apt-get install linux-headers-`uname -r`