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

acinclude.m4

@@ -1385,7 +1385,7 @@ AC_DEFUN([CURL_CHECK_WIN32_LARGEFILE], [
       AC_COMPILE_IFELSE([
         AC_LANG_PROGRAM([[
         ]],[[
-          #if !defined(_WIN32_WCE) && (defined(__MINGW32__) || defined(_MSC_VER))
+          #if !defined(_WIN32_WCE) && defined(__MINGW32__)
             int dummy=1;
           #else
             #error Win32 large file API not supported.
@@ -1396,18 +1396,7 @@ AC_DEFUN([CURL_CHECK_WIN32_LARGEFILE], [
       ])
     fi
     if test "$curl_win32_file_api" = "no"; then
-      AC_COMPILE_IFELSE([
-        AC_LANG_PROGRAM([[
-        ]],[[
-          #if defined(_WIN32_WCE) || defined(__MINGW32__) || defined(_MSC_VER)
-            int dummy=1;
-          #else
-            #error Win32 small file API not supported.
-          #endif
-        ]])
-      ],[
-        curl_win32_file_api="win32_small_files"
-      ])
+      curl_win32_file_api="win32_small_files"
     fi
   fi
   case "$curl_win32_file_api" in
@@ -1418,8 +1407,6 @@ AC_DEFUN([CURL_CHECK_WIN32_LARGEFILE], [
       ;;
     win32_small_files)
       AC_MSG_RESULT([yes (large file disabled)])
-      AC_DEFINE_UNQUOTED(USE_WIN32_SMALL_FILES, 1,
-        [Define to 1 if you are building a Windows target without large file support.])
       ;;
     *)
       AC_MSG_RESULT([no])