Add Print::availableForWrite

teensy3/Print.h

 	virtual size_t write(uint8_t b) = 0;
 	size_t write(const char *str)			{ return write((const uint8_t *)str, strlen(str)); }
 	virtual size_t write(const uint8_t *buffer, size_t size);
+	virtual int availableForWrite(void)		{ return 0; }
 	virtual void flush()				{ }
 	size_t write(const char *buffer, size_t size)	{ return write((const uint8_t *)buffer, size); }
 	size_t print(const String &s);

teensy3/usb_seremu.h

         size_t write(long n) { return write((uint8_t)n); }
         size_t write(unsigned int n) { return write((uint8_t)n); }
         size_t write(int n) { return write((uint8_t)n); }
-	int availableForWrite() { return usb_seremu_write_buffer_free(); }
+	virtual int availableForWrite() { return usb_seremu_write_buffer_free(); }
 	using Print::write;
         void send_now(void) { usb_seremu_flush_output(); };
         uint32_t baud(void) { return 9600; }

teensy3/usb_serial.h

 	size_t write(long n) { return write((uint8_t)n); }
 	size_t write(unsigned int n) { return write((uint8_t)n); }
 	size_t write(int n) { return write((uint8_t)n); }
-	int availableForWrite() { return usb_serial_write_buffer_free(); }
+	virtual int availableForWrite() { return usb_serial_write_buffer_free(); }
 	using Print::write;
         void send_now(void) { usb_serial_flush_output(); }
         uint32_t baud(void) { return usb_cdc_line_coding[0]; }