windows: drop redundant `USE_WIN32_SMALL_FILES` macro

windows: drop redundant `USE_WIN32_SMALL_FILES` macro

In effect it meant `_WIN32 && !USE_WIN32_LARGE_FILES`.
Replace it with these macros.

Also:
- configure: delete tautological check for small file support.
- configure: delete stray `_MSC_VER` reference. autotools does not
  support MSVC.
- drop tautological checks for WinCE in `config-win32*.h` when setting
  `USE_WIN32_LARGE_FILES`.
- merge related PP logic.
- prefer `#ifdef`, fix whitespace.

Suggested-by: Marcel Raad
Report: https://github.com/curl/curl/pull/15952#issuecomment-2580092328

Closes #15968