Building for macos now requires at least macos 10.12 SDK

[Mike Hommey]

Hi,

Automation was recently updated to use the 10.12 SDK, because it turns
out it had been a while we haven't supported running Firefox on macos
10.11 and earlier (support dropped in bug 1634765, SDK updated in bug
1680152).

As usual in those cases, but quicker than usual, something else landed
that made the 10.12 SDK implicitly required, by using an API new to
10.12 (bug 1678174).

The error message when building with the 10.11 SDK looks like:
"error: use of undeclared identifier 'CLOCK_REALTIME'"

It's only fair not to maintain code that users would never use in
practice just to keep things building with the older SDK, so we're going
to deprecate building with the 10.11 SDK.

If you've been using that SDK, you can remove the --with-macos-sdk line
from your mozconfig (or however other config you used) and use whatever
SDK you have with Xcode or the Command Line Tools (the build should pick
one on its own by default). There used to be problems with SDKs 10.13 or
10.14 and newer, but those are long gone. We're now even shipping
builds for arm64 based on the very latest 11.0 SDK.

Only in rare occasions would I expect the SDK version used in local
builds to matter, but if it does matter for your use-cases, you'll
unfortunately have to get the 10.12 SDK by your own means, from e.g.
Xcode 8.3.3.

Mike