Not defining USBCON broke since 1.8.14

We have a PlatformIO project which has a custom board without USB. For this we don't have USBCON defined which worked fine in v1.8.13

Since PR #713 this broke because USBAPI.h now uses LineInfo which is not defined.

The definition for LineInfo is in CDC.h , surrounded by an ifdef CDC_ENABLED (which is false, since USBCON is not defined), however since CDC.cpp does always include USBAPI.h this problem arises;

I originally created this issue in https://github.com/platformio/platform-atmelsam/issues/234 , but this indeed not an issue in PlatformIO but ArduinoCore

Start with CDC.cpp and inspect its inclusion of USBAPI.h, then trace the LineInfo definition in CDC.h and the CDC_ENABLED condition. Reproduce the build for a custom board without USBCON, and verify it succeeds while the existing USB-enabled configuration still builds.

Written by the indexing model from the issue text.