MSVC doesn't have isascii(), but it does have _isascii().
diff --git a/portability.h b/portability.h
index b361254..6786d57 100644
--- a/portability.h
+++ b/portability.h
@@ -88,6 +88,8 @@
 #endif
 
 #ifdef _MSC_VER
+  #define isascii	_isascii
+
   /*
    * If <crtdbg.h> has been included, and _DEBUG is defined, and
    * __STDC__ is zero, <crtdbg.h> will define strdup() to call