Using the CMake build on WIN32, the tif_win32.c does not get used but
tif_unix.c is used instead.

In the top CMakeLists.txt file we have:

--8<--
# Win32 IO
set(win32_io FALSE)
if(WIN32)
  set(win32_io TRUE)
endif()

set(USE_WIN32_FILEIO ${win32_io})
--8<--

In the libtiff CMakeLists.txt file we have:

--8<--
if(WIN32_IO)
  extra_dist(tif_unix.c)
  list(APPEND tiff_SOURCES tif_win32.c)
else()
  extra_dist(tif_win32.c)
  list(APPEND tiff_SOURCES tif_unix.c)
endif()
--8<--

So the cause is that the wrong test is done.  Casing in CMake variable names
matter.  Changing "if(WIN32_IO)" to "if(win32_io)" solves this problem.