Fix Yield when no Serial on USB

teensy3/usb_serial.h

 #ifndef USBserial_h_
 #define USBserial_h_
 
-#if defined(USB_SERIAL) || defined(USB_SERIAL_HID)
+#if (F_CPU >= 20000000) && (defined(USB_SERIAL) || defined(USB_SERIAL_HID))
 
 #include <inttypes.h>
 
-#if F_CPU >= 20000000
-
 // C language implementation
 #ifdef __cplusplus
 extern "C" {
 #endif // __cplusplus
 
 
-#else  // F_CPU < 20 MHz
+#else  // F_CPU < 20 MHz && !(USB_SERIAL || USB_SERIAL_HID)
 
 // Allow Arduino programs using Serial to compile, but Serial will do nothing.
 #ifdef __cplusplus
 extern void serialEvent(void);
 #endif // __cplusplus
 
-#endif // F_CPU
-
-#endif // USB_SERIAL || USB_SERIAL_HID
+#endif // F_CPU && USB_SERIAL || USB_SERIAL_HID
 
 #endif // USBserial_h_