Force Cygwin to use function tolower() rather than macro version

cyassl/ctaocrypt/types.h

@@ -217,6 +217,11 @@ enum {
         #define XISALPHA(c)     isalpha((c))
     #endif
     /* needed by CyaSSL_check_domain_name() */
+    #ifdef __CYGWIN__
+        /* Cygwin uses a macro version of tolower() by default, use the
+         * function version. */
+        #undef tolower
+    #endif
     #define XTOLOWER(c)      tolower((c))
 #endif