From "[PATCH 09/13] ntdll: Import valgrind headers for PE side ntdll.":
One of the motivating factors for 9951f2f43 was to allow including headers like valgrind from PE code. I'm not immediately sure how to solve the problem of detecting the header, but it doesn't seem obviously intractable, and potentially preferable to do that rather than import more actively developed external code into the project.