Add build tests to verify correct compilation of as many modules as possible under as many architectures as possible.

Fix broken compilation of LUFA under C++ compilers when the Serial peripheral module header file is included in a C++ source file.

Fix missing semicolon in the UC3 architecture host pipe functions.

Fix failed compilation for the XMEGA architecture if USB_DEVICE_ONLY us not specified.

LUFA/Drivers/USB/Core/USBTask.c

@@ -52,8 +52,10 @@ void USB_USBTask(void)
 	#else
 		if (USB_CurrentMode == USB_MODE_Device)
 		  USB_DeviceTask();
+		#if defined(USB_CAN_BE_HOST)
 		else if (USB_CurrentMode == USB_MODE_Host)
 		  USB_HostTask();
+		#endif
 	#endif
 }