warning-free zephyr builds

[pcppcp]

• [ ] use #pragma to silence warnings that come from zephyr includes
• [ ] enable -Werror for zephyr builds

[vishav26]

Hi,

I saw this on cmake documentation. Add -Werror=dev while building zephyr. https://cmake.org/cmake/help/v3.12/manual/cmake.1.html

[pcppcp]

that's completely unrelated; that flags is meant to raise errors when parsing cmakelists.txt files

[pcppcp]

Fixed by #144