In my engine I support Win7, Win8RT, OSX, Android, iOS, Web (JS+WebGL), NaCl and NPAPI, but not linux. The reason is majorly because the number of distros is huge and after evaluating it, the build and packaging process is a pain. If you ask five different linux people what the most important distro to support is, you'll get at least five different answers. Depending on distro * distro version * kernel version * GPU card vendor combination, the support for OpenGL driver varies wildly. The system testing complexity is up the roof compared to any other platform. And most importantly, since the market segment is smaller compared to Windows and OSX and there are no good marketing channels, I can't see the point in it. If you're already a linux whiz that knows the different distros and kernels and drivers in and out, perhaps you'll be able to pull off decent support for all the combinations with a bearable/manageable pain, but for "normal" developers, I don't think it's at all worth it.
If some big player (Valve+Steam?) comes in and manages to unify the development pain (doubt it), then I'll definitely be reconsidering.
You may find these articles a good source, supposedly they explain how to create an exe that will run and install on any distro, so you only have to create and manage one.