This reverts commit 476e33c3f3632bd32370fadc67b10d61da9a4098.
We’ve decided to remove `G_OS_DARWIN` in favour of recommending people
use `__APPLE__` instead. As per the discussion on #2802 and linked
issues,
* Adding a new define shifts the complexity from “which of these
platform-provided defines do I use” to “which platform-provided
defines does G_OS_DARWIN use”
* There should ideally be no cases where a user of GLib has to use
their own platform-specific code, since GLib should be providing
appropriate abstractions
* Providing a single `G_OS_DARWIN` to cover all Apple products (macOS
and iOS) hides the complexity of what the user is actually testing:
are they testing for the Mach kernel, the Carbon and/or Cocoa user
space toolkits, macOS vs iOS vs tvOS, etc
Helps: #2802
7b52ccbfc4