Update, 2021
January 2021. Since writing about Xindy below in 2016, a new indexing program has been released, Xindex by Herbert Voß. I now use xindex with this configuration file for IAST sorting. My preamble says
\usepackage[imakeidx]{xindex}
\makeindex[name=lexical,
title=Lexical Index,
columns=3,
options= -c iast -a -n, % nocasesensitive, noheadings
]
