Don't use %zx because it's unsupported on mingw64

[oli-obk]

Also check that no warnings happen on travis

cc #569

[xor-gate]

Looks good i'm only not 100% sure if clang or other strict compiler will eat this. You should prefer adding a size to the cast so:

(unsigned) becomes (unsigned int)

Then we are good to go.

[oli-obk]

Ok... so after this PR:

• travis will check gcc and clang on both linux and os-x
• all compilations must be warning free
• O2 is used instead of O3 optimizations (this was intended by the CMakeLists.txt, but never actually called due to elseif() not doing the same as else()

i'm only not 100% sure if clang or other strict compiler will eat this.

clang accepted it, but I still made the change.

Maybe there's a warning we can enable to enforce this?

[xor-gate]

Thank you for this improvement and understanding!

[xor-gate]

I'm not sure, probably GCC or some analyser have warned me some day about no defined typed, assuming int.