The test uses functions from C that were introduced in OSX 1.12.
Include stubs for those functions when compiling for 1.11 and earlier.
This test really a compile-time test, it doesn't matter much what the
executed code actually does.
Use a nasty #define hack to work around the fact that cgo doesn't
support static global variables.

Update #24161
Fixes #26355

Change-Id: Icf6f7bc9b6b36cacc81d5d0e033a2ebaff7e0298
Reviewed-on: https://go-review.googlesource.com/123715
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>