右键一个Project，可以发现有两个地方设置Include的相关目录：
1. VC++ Directories -> Include Directories
2. C/C++ -> General -> Additional Include Directories
不知道具体有什么区别，后在MSDN上得到答案：
“VC++ Directories -> Include Directories” : Directory settings displayed in the window are the directories that Visual Studio will search for include files referred to in your source code files. Corresponds to environment variable INCLUDE. More information : http://msdn.microsoft.com/en-us/library/t9az1d21(v=vs.80).aspx
“C/C++ -> General -> Additional Include Directories”: The directory to be added to the list of directories searched for include files. More information : http://msdn.microsoft.com/en-us/library/73f9s62w(v=vs.80).aspx
下面那个链接讲的比较清楚了：
The compiler searches for directories in the following order:
1.Directories containing the source file.
2.Directories specified with the /I option, in the order that CL encounters them.
3.Directories specified in the INCLUDE environment variable.
order2中的/I是由C/C++ -> General -> Additional Include Directories设置的。
order3中的INCLUDE是由VC++ Directories -> Include Directories设置的。