Handle older macOS SDKs not defining TARGET_OS_OSX.

Change-Id: I4a0566f3f1cb2d6320e3de80619aa90e0f53ff27
Reviewed-on: https://code-review.googlesource.com/c/re2/+/59690
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed