gnulib is doing this to make fflush() behaviour to match what they expect.
It accesses internal of __sFILE to change behavior of fseeko(), but we are
hiding internal of __sFILE and our fseeko() is compatible with what gnulib
expects already.
Then the patch is to just use our fseeko().

Index: src/gl/fseeko.c
--- src/gl/fseeko.c.orig
+++ src/gl/fseeko.c
@@ -76,6 +76,8 @@ fseeko (FILE *fp, off_t offset, int whence)
                      ? fp_->_bf._size
                      : 0)
       && fp_ub._base == NULL)
+#elif defined __OpenBSD__ /* recent version of OpenBSD */
+  if (0)	/* fseeko() is compatible with what gnulib expects */
 #elif defined __EMX__               /* emx+gcc */
   if (fp->_ptr == fp->_buffer
       && fp->_rcount == 0
