A problem regarding importing symlinked modules was recently reported on the
Cygwin mailing list:

    http://cygwin.com/ml/cygwin/2005-04/msg00257.html

The following test case demonstrates the problem:

$ ls -l
total 1
lrwxrwxrwx    1 jt       None            6 Apr 23 13:32 bar.py -> foo.py
-rw-r--r--    1 jt       None           24 Apr 18 20:13 foo.py

$ python -c 'import bar'
Traceback (most recent call last):
File "<string>", line 1, in ?
ImportError: No module named bar

Since Cygwin's case_ok() uses a modified version of the Windows's version, the
symlinked bar module actually resolves to file foo.py instead of bar.py. This
obviously causes the matching code to fail (regardless of case).

The patch fixes this problem by making Cygwin use the Mac OS X case_ok()
instead of a modified Window's version.