How to include other package's headers?

edit

I tried to create "A" and "B" package.

I added include/A/a.h on directory which "roscd A".

Then I use a.h in b.cpp and add manifest on package B:

 #include <A/a.h>

 int main()
 {
    return 0;
 }

manifest:

 <package>
   <description brief="B">

      B

   </description>
   <author>sam</author>
   <license>BSD</license>
   <review status="unreviewed" notes=""/>
   <url>http://ros.org/wiki/B</url>
   <depend package="roscpp"/>
   <depend package="A"/>

 </package>

But compile failed because of couldn't find A/a.h.

How to solve it?

Thank you~