でびまるです。もう遅いでしょうが Ubuntu 14.10 だと問題ないようです。
$ cat /etc/issue.net
Ubuntu 14.10
$ mendex -f -U sample.idx
This is mendex version 2.6f [14-Aug-2009] (utf8.uptex) (TeX Live 2014).
Scanning input file sample.idx....done (496 entries accepted, 0 rejected).
496 entries accepted, 0 rejected.
Sorting index....done(4545 comparisons).
Sorting pages....done(33 comparisons).
Making index file....done.
0 warnings, written in sample.ilg.
Output written in sample.ind.
参考まで。