Specifically, std.fs.OpenDirOptions struct no longer has the iterable
flag, now it's a standalone function call std.fs.openIterableDir

* Add zig buildfile for examples.

- `zig build` to compile all examples
- `zig build [module]` to compile all examples for a module (e.g. `zig build core`)
- `zig build [example]` to compile _and run_ a particular example (e.g. `zig build core_basic_window`)

You can use `-Dtarget=` to compile for a non-native platform, such as
`zig build -Dtarget=x86_64-windows-gnu` to compile from Linux to
Windows.

* Skip pthread example on Windows.

* Select appropriate lib file based on target.