`"unix"` or `"windows"`. The value of this configuration option is defined
as a configuration itself, like `unix` or `windows`.
* `target_os = "..."`. Operating system of the target, examples include
- `"win32"`, `"macos"`, `"linux"`, `"android"`, `"freebsd"`, `"dragonfly"`,
+ `"windows"`, `"macos"`, `"ios"`, `"linux"`, `"android"`, `"freebsd"`, `"dragonfly"`,
`"bitrig"` or `"openbsd"`.
* `target_pointer_width = "..."`. Target pointer width in bits. This is set
to `"32"` for targets with 32-bit pointers, and likewise set to `"64"` for