Proton SDK is Seth's GL/GLES messy multi-platform C++ game SDK. Can output to Windows, Linux (including the Raspberry Pi), HTML5, OS X, iOS, Android. Free and open source.
A component based toolbox of useful things built up over the last ten years. Instead of a giant .lib you link only the .cpp files used when possible to simplify multiplatform support as well as keep code size down.
It's kind of an SDL-like on steroids (while also being able to target SDL2 for setup/input/audio itself when needed) but generally gets the best results with its own native implementations of things. For example, it can target the following audio subsystems: SDL2_mixer, Audiere, FMOD, FMODStudio, Native iOS, Native Android, Denshion, Native Flash
It's designed with a “Write stuff in Windows with Visual Studio 2017, then compile/export to other platforms as needed” mentality, although you could certainly use alternate IDEs and platforms to develop on. (Xcode projects are available but only CMake setups for most other platforms)
Deprecated technologies/platforms no longer actively supported: Flash, BBX, WebOS, Irrlicht, Bullet