Missing SDL_ttf.h

Try: sudo apt-get install libsdl2-ttf-dev

After that, you should find it via:

find /usr|grep SDL_ttf

I was installing gosu gem for Ruby and had the same error, on Ubuntu 16.04. All solutions pointed to installing sdl2 ttf libs, but I already had them all installed. I noticed, however, that I had /usr/include/SDL2/SDL_ttf.h, and the gem included lib directories didn't list that one, but listed /usr/local/include/SDL2/SDL_ttf.h.

I managed to solve the problem by creating a symbolic link with

sudo ln -s /usr/include/SDL2/SDL_ttf.h /usr/local/include/SDL2/