Linking to Libraries

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I had the same problem. The problem is the compile line be sure
to also include (without backquotes) something like: -lSDL_gfx    e.g. here is my build line from Geany:
gcc -Wall  `sdl-config --cflags --libs` -lSDL_gfx -lm -o "%e" "%f";
where %e is the compiled name and %f is the object file name.  Seems the documentation is fragmented.
One alternative is cairo graphics for better control, smoothing, and registered primitives.
In example I found they had included type bool, not in GNU C! Quick fix:  int true=1,false=0;