Regarding building C/C++ … I don’t know of a package that offers a build solution for C/C++ programs. That doesn’t mean it doesn’t exist … just that I haven’t seen it or don’t remember it being mentioned here.
Atom does not yet have a unified build system like Sublime Text does. Though perhaps someone has built a package that emulates this.
C/C++ isn’t built in such a way that it can be easily runtime evaluated, like the Run in Atom package. For this, I think you would have to go to a dedicated C/C++ IDE.
Please specify what it is you’re looking for that is missing in the C/C++ syntax highlighting and what you’re really looking for when you say “Auto-complete”.
I suspect that you’re looking for a C/C++ IDE and, as mentioned in other topics on the board, while Atom has some IDE-like features for specific languages because of some of the great community members that have made these things possible … Atom is an editor, not an IDE. Perhaps Atom will get to the point where it will have more comprehensive IDE-like features (an Intellisense framework for any language, comprehensive syntactic, semantic and style checking, code reformatting, etc) … but that is a ways off at this point, I believe.