This looks quite nice. But they did fib a bit about no dependencies. It is apparently built on top of GLFW3 which is a nice, small C library that gives you a native window and input handling.
$ make
gcc main.c -std=c89 -pedantic -O2 -D_POSIX_C_SOURCE=200809L -o bin/zahnrad -lX11 -lm
$ ldd bin/zahnrad
linux-vdso.so.1 (0x00007ffe32df9000)
libX11.so.6 => /usr/lib/libX11.so.6 (0x00007f8468746000)
libm.so.6 => /usr/lib/libm.so.6 (0x00007f8468441000)
libc.so.6 => /usr/lib/libc.so.6 (0x00007f84680a0000)
libxcb.so.1 => /usr/lib/libxcb.so.1 (0x00007f8467e7d000)
libdl.so.2 => /usr/lib/libdl.so.2 (0x00007f8467c79000)
/lib64/ld-linux-x86-64.so.2 (0x00007f8468a88000)
libXau.so.6 => /usr/lib/libXau.so.6 (0x00007f8467a75000)
libXdmcp.so.6 => /usr/lib/libXdmcp.so.6 (0x00007f846786f000)