'fflush' test failure on Cygwin

[Bruno Haible]

Hi Eric,

On Cygwin 2006 (/usr/bin/cygwin1.dll dated 2006-01-20), the test-fflush test
fails:

  File offset is wrong.
  FAIL: test-fflush.exe

In test-fflush.c line 70, the return value from lseek() is 10 instead of the
expected 5.

Bruno