C# together with visual studio solves this in an amazing way. You can browse .NET DLL-files as if they were source files, with only public members being visible and without the function bodies. Good for finding function names and constants.
I can't believe nobody else is using the same system, using "go to definition" on a library function is just as natural as using it on one of your own functions. Only difference is in the second case you see the actual code of the function instead of just the function header.
(Replace .NET DLL-files with these new "modules" to make this applicable to C)
Ps. even if you do have the source, in most editors you can collapse the code to hide everything so you don't have to "slog through all the source".
Thinking about my trips to /usr/include, those headers weren't that useful for coding with but you could get constants and function names at least.