Please bear in mind that WINDOWSNT is defined when Emacs is built for
native Windows. WINDOWSNT says nothing about the GUI. A WINDOWSNT
without a window system is reasonable. HAVE_NTGUI, on the other hand,
is defined when Emacs is being built for the traditional Windows UI
toolkit. While these two macros are usually defined together, they
don't have to be. It's conceivable that we might support a GTK+
WINDOWSNT Emacs or a Wine-based FreeBSD Emacs HAVE_NTGUI Emacs linked
against Wine. Please don't conflate the two macros.