patch 8.0.1704: 'backupskip' default doesn't work for Mac

Problem:    'backupskip' default doesn't work for Mac.
Solution:   Use "/private/tmp". (Rainer Müller, closes #2793)
diff --git a/src/option.c b/src/option.c
index 48432bb..5ef346c 100644
--- a/src/option.c
+++ b/src/option.c
@@ -3367,7 +3367,11 @@
 	    mustfree = FALSE;
 # ifdef UNIX
 	    if (*names[n] == NUL)
+#  ifdef MACOS_X
+		p = (char_u *)"/private/tmp";
+#  else
 		p = (char_u *)"/tmp";
+#  endif
 	    else
 # endif
 		p = vim_getenv((char_u *)names[n], &mustfree);