[tlbuild] mendexk test failure on Cygwin

[Ken Brown]

The mendexk test fails on Cygwin as follows:

$ cat ~/src/texlive/test.x86_64/Work/texk/mendexk/test-suite.log
==============================================
    mendex (TeX Live) 2.6f: ./test-suite.log
==============================================

# TOTAL: 1
# PASS:  0
# SKIP:  0
# XFAIL: 0
# FAIL:  1
# XPASS: 0
# ERROR: 0

.. contents:: :depth: 2

FAIL: tests/mendex
==================

This is mendex version 2.6f [14-Aug-2009] (utf8.euc) (TeX Live 2015/dev).
Scanning input file ../../../texk/mendexk/tests/foo.idx....done (3 entries 
accepted, 0 rejected).
3 entries accepted, 0 rejected.
Sorting index....done(0 comparisons).
Sorting pages....done(4 comparisons).
Making index file....done.
0 warnings, written in foo.ilg1.
Output written in foo.ind1.
3c3
<   \item foo, \mac{1}, 1
---
 >   \item foo,


A bisection shows that the problem starts with revision 34287.  Specifically, 
reverting the change to texk/mendexk/fwrite.c in that revision fixes the 
problem.  Let me know what further information I can provide to help track this 
down.

Ken