I don't believe you can extract the contents of a cmd window without somehow using DllCall to read the process memory directly.
If you just want the output of a CLI command such as Grep or AWK, using stdout via the run command should work. Honestly though, I stopped relying on AHK because this sort of thing is just too clunky.
http://www.autohotkey.com/docs/commands/Run.htm.
Edit for comments:
What you want is doable, but the solution depends entirely on how your IDE works. What behavior does it have that's unique to building a project? If it makes temp files, you can overload your "build" button with an AHK subroutine that watches for the existence of those files, and then checks the modified date of the output executable to see if the build succeeded. The same kind of solution works if the IDE changes its window title when building. Be clever. :)
Failing that, you might have to install a message hook.