Hi there,
I do not have LPC1768 but I’m working on the Nucleo-F429ZI with USB OTG and it is working.
The USBSerial is set to blocking in the default. That mean it waits for a connection from opposite side, that is the reason why your code not reach the main. So until you not connect a terminal application on your pc to a correct port nothing will happen.
The PC application also need manage the DTR signal.
Also you can not call _printc for call a string without a for loop, the _printc(int c) means print only one char.
When you want a standart serial connection with PC you need to check BufferedSerial or UnbufferedSerial.
BR, Jan