[stable] Comment out prototypes for OSX symbols missing on < 10.12

[ibuclaw]

Phobos uses these (guarded by ie: static if (is(typeof(&utimensat)))), which causes a regression in building complete compiler and runtime.

Preferably we should just add a version mechanism like __traits(getTargetInfo, "macOS_version") - ditto for freebsd version too. But to just unbreak the release, just leave these left for now.

[dlang-bot]

Thanks for your pull request, @ibuclaw!

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + dmd#22978"

[Herringway]

Perhaps this __traits(getTargetInfo) could return a struct with a version field.

[ibuclaw]

This is strictly a regression fix, as the prototype didn't exist before in older versions of druntime.