Fix for https://github.com/cstawarz

Author: ladyada

cores/arduino/Print.h

@@ -56,6 +56,10 @@ class Print
       return write((const uint8_t *)buffer, size);
     }
 
+    // default to zero, meaning "a single write may block"
+    // should be overriden by subclasses with buffering
+    virtual int availableForWrite() { return 0; }
+
     size_t print(const __FlashStringHelper *);
     size_t print(const String &);
     size_t print(const char[]);
@@ -82,6 +86,8 @@ class Print
     size_t println(void);
 
     void printf(const char[], ...);
+
+    virtual void flush() { /* Empty implementation for backward compatibility */ }
 };
 
 #endif