patch 8.0.1166: :terminal doesn't work on Mac High Sierra

Problem:    :terminal doesn't work on Mac High Sierra.
Solution:   Change #ifdef for OpenPTY(). (Ozaki Kiichi, Kazunobu Kuriyama,
            closes #2162)
diff --git a/src/pty.c b/src/pty.c
index 9b03612..151a278 100644
--- a/src/pty.c
+++ b/src/pty.c
@@ -267,10 +267,11 @@
 }
 #endif
 
-#if defined(HAVE_SVR4_PTYS) && !defined(PTY_DONE) && !defined(hpux) && !defined(MACOS_X)
+#if defined(HAVE_SVR4_PTYS) && !defined(PTY_DONE) && !defined(hpux) \
+                    && !(defined(MACOS_X) && !defined(MAC_OS_X_VERSION_10_6))
 
 /* NOTE: Even though HPUX can have /dev/ptmx, the code below doesn't work!
- * Same for Mac OS X Leopard. */
+ * Same for Mac OS X Leopard (10.5). */
 #define PTY_DONE
     int
 OpenPTY(char **ttyn)