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/if_perl.xs b/src/if_perl.xs
index ecb9726..076c5c6 100644
--- a/src/if_perl.xs
+++ b/src/if_perl.xs
@@ -1288,7 +1288,7 @@
     linenr_T	i;
     buf_T	*was_curbuf = curbuf;
 
-    if (bufempty())
+    if (BUFEMPTY())
 	return;
 
     if (perl_interp == NULL)