Disable -Wunused-but-set-variable (used by gcc) for the tutorials.
Disable -Wunused-but-set-variable (used by gcc) for the tutorials.

Several tutorials do this when demonstrating the C code equivalent of a
function. Using a #pragma to disable isn't really an option because

- We want to keep the tutorial code clean
- Unrecognised pragmas will cause other compilers to emit unrecognised
  pragma warnings (e.g. clang)


Former-commit-id: 70633dc0ac8dbecb5ce791c15b51437364fc40cb
Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide