towlower is defined by wctype.h

--- a/src/multibyte.h
+++ b/src/multibyte.h
@@ -2,6 +2,7 @@
 
 #ifdef MULTIBYTE
 #include <wchar.h>
+#include <wctype.h>
  #define CHAR wchar_t
  #define STRLEN(s) wcslen(s)
  #define CPYSS(dest, src) wcscpy(dest, src) /* copy str to str */