Significance of !HAVE_LIBC dependency in Win32 thread function

What is the significance of !HAVE_LIBC dependency in the Win32
thread function? (e.g. see SDL_thread.h) Because HAVE_LIBC is
a curious thing: You can have a libc, but you can statically
link SDL to it, e.g. with MSVC or Watcom. Besides, there is no
guarantee that SDL and the client application are linked to the
very same libc.

So: is there an actual harm if HAVE_LIBC == 1, and SDL still
uses the SDL_PASSED_BEGINTHREAD_ENDTHREAD mechanism?