This was a MinGW-specific, non-MS-DDK/WDK-compatible define, that was used to mark NTOS kernel/hal exports, instead of NTSYSAPI etc. We have since fixed that, and changed the way Freeldr (and rossym) manages these, see commits: 186c8b72d (r16028), 51f0dfd30 (r17651) and 526efd2ee (r24359)