HoudahSpot is a Spotlight-replacement app, as is NotLight. HoudahSpot takes a different approach, being more similar to Finder and Spotlight, but with the suck subtracted from their implementations. (NotLight is basically a front end on the Spotlight APIs.)
It's fast, but buggy. The query above, for example, didn't work as long as I had any folders other than /Developer/SDKs listed. Perhaps it's using AND rather than OR on the source files? No idea, but as long as I can't search all my header folders at once, I won't be abandoning my own header search tool. UPDATE 17:11 PDT: NotLight doesn't find anything either. Spotlight does, though. durin42 and I both searched for fprintf as well; I had the same results as with readline, whereas durin42 got a hit in HoudahSpot for readline but not for fprintf (and remarked that “spotlight behaves oddly”). Very, very strange.
Also, the search window's minimum size is unnecessarily large. The window as shown above is as small as I could make it. Wasted space is bad. In a quasi-related misfeature, the toolbar cannot be configured. The maximum extent to its configurability is that you can ⌘-click the pill button to switch between Icon & Text and Icon-only. Yaaay. My last nitpick is that it doesn't use pretty plus/minus images, like mine.
If it gets down to free, snap it up; it shows a lot of promise, and is very pretty. But I would not pay $15 (or $9, the current BlogZOT! price as of this writing) for it when there's NotLight. I look forward to some future version that fixes all the bugs.