fseek and ftell with large files on Windows

[Markus Mützel]

Octave is using gnulib's "fseek" and "ftell" functions. A user reported that 
they have issues when using these functions with large files on Windows:
https://savannah.gnu.org/bugs/index.php?66399

If I understand the code paths for Windows correctly, the issue might be caused 
by the fact that the "fseek" and "ftell" functions are forwarding to 
"_fseeki64" or "_ftelli64", respectively, if the size of the "off_t" type is 
smaller than 64-bit. They are forwarding to "fseek" and "ftell" otherwise.
However, even if the "off_t" type is 64-bit (or larger), "fseek" and "ftell" 
still return "long" on Windows which is only 32-bit wide:
https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/ftell-ftelli64?view=msvc-170

The following change avoids the issue described in the above bug report for me. 
I don't know whether that change is correct though.

Thank you,
Markus

diff --git a/lib/fseeko.c b/lib/fseeko.c
index 2c3b053a3b..30f8f70afe 100644
--- a/lib/fseeko.c
+++ b/lib/fseeko.c
@@ -31,7 +31,7 @@ fseeko (FILE *fp, off_t offset, int whence)
 # undef fseek
 # define fseeko fseek
 #endif
-#if _GL_WINDOWS_64_BIT_OFF_T
+#if _WIN32
 # undef fseeko
 # if HAVE__FSEEKI64 && HAVE_DECL__FSEEKI64 /* msvc, mingw since 
msvcrt8.0, mingw64 */
 #  define fseeko _fseeki64
diff --git a/lib/ftello.c b/lib/ftello.c
index 88247bca8e..2f83c61c0e 100644
--- a/lib/ftello.c
+++ b/lib/ftello.c
@@ -34,7 +34,7 @@ ftello (FILE *fp)
 # undef ftell
 # define ftello ftell
 #endif
-#if _GL_WINDOWS_64_BIT_OFF_T
+#if _WIN32
 # undef ftello
 # if HAVE__FTELLI64 /* msvc, mingw64 */
 #  define ftello _ftelli64