patch 8.0.1688: some macros are used without a semicolon

Problem:    Some macros are used without a semicolon, causing auto-indent to be
            wrong.
Solution:   Use the do-while(0) trick. (Ozaki Kiichi, closes #2729)
diff --git a/src/option.c b/src/option.c
index 084e5e1..48432bb 100644
--- a/src/option.c
+++ b/src/option.c
@@ -8456,7 +8456,7 @@
     else if ((int *)varp == &p_acd)
     {
 	/* Change directories when the 'acd' option is set now. */
-	DO_AUTOCHDIR
+	DO_AUTOCHDIR;
     }
 #endif