Don't define _XOPEN_SOURCE on Apple platforms

This breaks the build on recent versions of macOS. Defining it makes
__DARWIN_C_LEVEL equal to _POSIX_C_SOURCE, which disables access to
functions like timespec_get and aligned_alloc on macOS 10.15 and newer,
which are now used by libc++.

Change-Id: I9215aaf203f19fbe045431d9ace160d588864c43
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/icu/+/419140
Reviewed-by: Roland McGrath <mcgrathr@google.com>
1 file changed