patch 8.0.0448: some macros are in lower case

Problem:    Some macros are in lower case, which can be confusing.
Solution:   Make a few lower case macros upper case.
diff --git a/src/option.c b/src/option.c
index 2e2f0f1..2c4b1c2 100644
--- a/src/option.c
+++ b/src/option.c
@@ -4170,7 +4170,7 @@
     }
 #endif
 
-    if (bufempty())
+    if (BUFEMPTY())
     {
 	int idx_ffs = findoption((char_u *)"ffs");