It is sometimes useful to skip generating of the header file (e.g. https://github.com/ziglang/zig/issues/2173), and zig compiler provides an option `--disable-gen-h` to control that behaviour. However, setting `lib.disable_gen_h = true` in a typical `build.zig` didn't append the option to arguments. This commit fixes it and adds a convenient `setDisableGenH` setter.