patch 8.0.0751: OpenPTY missing with some combination of features

Problem:    OpenPTY missing with some combination of features. (Kazunobu
            Kuriyama)
Solution:   Adjust #ifdef.  Also include pty.pro when needed.
diff --git a/src/misc2.c b/src/misc2.c
index 7014e16..541069e 100644
--- a/src/misc2.c
+++ b/src/misc2.c
@@ -6050,6 +6050,9 @@
 }
 
 # ifdef USE_VIMPTY_GETENV
+/*
+ * Used for mch_getenv() for Mac.
+ */
     char_u *
 vimpty_getenv(const char_u *string)
 {