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

lib/config-win32.h

@@ -387,22 +387,13 @@ Vista
 /*                        LARGE FILE SUPPORT                        */
 /* ---------------------------------------------------------------- */
 
-#if defined(_MSC_VER) && !defined(_WIN32_WCE)
+#if defined(_MSC_VER) || defined(__MINGW32__)
 #  define USE_WIN32_LARGE_FILES
-#endif
-
-#if defined(__MINGW32__) && !defined(USE_WIN32_LARGE_FILES)
-#  define USE_WIN32_LARGE_FILES
-#endif
-
-#if !defined(USE_WIN32_LARGE_FILES) && !defined(USE_WIN32_SMALL_FILES)
-#  define USE_WIN32_SMALL_FILES
-#endif
-
 /* Number of bits in a file offset, on hosts where this is settable. */
-#if defined(USE_WIN32_LARGE_FILES) && defined(__MINGW32__)
-#  ifndef _FILE_OFFSET_BITS
-#  define _FILE_OFFSET_BITS 64
+#  ifdef __MINGW32__
+#    ifndef _FILE_OFFSET_BITS
+#    define _FILE_OFFSET_BITS 64
+#    endif
 #  endif
 #endif
 

lib/config-win32ce.h

@@ -232,13 +232,7 @@
 /*                        LARGE FILE SUPPORT                        */
 /* ---------------------------------------------------------------- */
 
-#if defined(_MSC_VER) && !defined(_WIN32_WCE)
-#  define USE_WIN32_LARGE_FILES
-#endif
-
-#if !defined(USE_WIN32_LARGE_FILES) && !defined(USE_WIN32_SMALL_FILES)
-#  define USE_WIN32_SMALL_FILES
-#endif
+/* Windows CE does not support large files */
 
 /* ---------------------------------------------------------------- */
 /*                           LDAP SUPPORT                           */

lib/curl_multibyte.c

@@ -32,7 +32,7 @@
 
 #include "curl_setup.h"
 
-#if defined(_WIN32)
+#ifdef _WIN32
 
 #include "curl_multibyte.h"
 
@@ -84,10 +84,6 @@ char *curlx_convert_wchar_to_UTF8(const wchar_t *str_w)
   return str_utf8;
 }
 
-#endif /* _WIN32 */
-
-#if defined(USE_WIN32_LARGE_FILES) || defined(USE_WIN32_SMALL_FILES)
-
 /* declare GetFullPathNameW for mingw-w64 UWP builds targeting old windows */
 #if defined(CURL_WINDOWS_UWP) && defined(__MINGW32__) && \
   (_WIN32_WINNT < _WIN32_WINNT_WIN10)
@@ -329,7 +325,7 @@ int curlx_win32_stat(const char *path, struct_stat *buffer)
       target = fixed;
     else
       target = path_w;
-#if defined(USE_WIN32_SMALL_FILES)
+#ifndef USE_WIN32_LARGE_FILES
     result = _wstat(target, buffer);
 #else
     result = _wstati64(target, buffer);
@@ -343,7 +339,7 @@ int curlx_win32_stat(const char *path, struct_stat *buffer)
     target = fixed;
   else
     target = path;
-#if defined(USE_WIN32_SMALL_FILES)
+#ifndef USE_WIN32_LARGE_FILES
   result = _stat(target, buffer);
 #else
   result = _stati64(target, buffer);
@@ -354,4 +350,4 @@ int curlx_win32_stat(const char *path, struct_stat *buffer)
   return result;
 }
 
-#endif /* USE_WIN32_LARGE_FILES || USE_WIN32_SMALL_FILES */
+#endif /* _WIN32 */

lib/curl_setup.h

@@ -497,7 +497,7 @@
  * Small file (<2Gb) support using Win32 functions.
  */
 
-#ifdef USE_WIN32_SMALL_FILES
+#if defined(_WIN32) && !defined(USE_WIN32_LARGE_FILES)
 #  include <io.h>
 #  include <sys/types.h>
 #  include <sys/stat.h>