My project was working without errors on XCode 4.2 on Lion. Now I'm running Mountain Lion with XCode 4.4 and when I try to build the project it gives me an error that tree.h file cannot be found. I checked the header search path and it was correct. I searched the web for such an error and found the same solution on many different pages but couldn't understand that particular solution, see for example this page.
How can I solve this problem? Alternately, can you clarify the weird solution that is spread all over the internet?
I had to add another header search path which is: $(SDKROOT)/usr/include/libxml2. This fixed the problem.