I’m not going to be departing from SDL2 any day soon, but if I were starting from scratch, I would seriously consider raylib.  It ticks many boxes!

• Written in C(C99). Tick.
• Cross platform including Raspberry Pi desktop. Tick.
• Open source and liberal licensing tick.
• Full 3D support with animated models. Tick.
• Extensive Sound support. Tick.
• Very open license that even allows static linking with closed software. Tick.
• Lots of examples. Tick.

There’s even a set of open source games on GitHub. including several that you can play in your browser (HTML5). Documntation is in the form of a 36-page Wiki. I took a quick glance through there and was impressed with some of the features. For example, OpenGl can be used directly and not through X11 though that is also available.

Plus full marks for including struct sizes on the data structures page. That’s not something you often see, nor is instructions for configuring Visual Studio, Visual Studio Cocde, Codeblocks, Eclipse and Sublime Text. The cheat sheet (which you can also download as a pdf) gives an idea of the number of functions in Raylib. They cover five pages!