Pass options from build.zig to source

Options can be passed from build.zig to any zig source file. This can be used for configuration and creating builds for a specific purpose, such as running extensive runtime tests.

The option is added by calling `addBuildOption(...)` on a `LibObjExeStep` (e.g. the result from `addObject(...)`, then used by importing zig-cache/{object name}_build_options.zig.