Need to understand terms pertaining to Includes Path + Widgets...

[OldFangle]

Hi
I've done one pass across Qt's help files and now I am trying to get a couple of the Examples working.
I'm getting the TargetConditionals.h error message.
I'm not understanding what I'm reading, about what to do when Qt says it can't find that file.
I've been trying to guess keywords to find information about how to tell Qt to look down the .../usr/includes path.
I've seen mention of Widgets but I'm new to "Widgets" and don't know specifically where to go or what to do when I get there, in order to tell Qt where to look for TargetConditionals.h so that I can at least get its Examples working.

I haven't been able to mentally build the whole overview yet, from what I've read. Can you help me fill in the blanks?

Thanks