Adding Tai Tham Script to GNU/Linux Distribution's Version of Emacs

[Richard Wordingham <richard.wordingham@ntlworld.com>]

I am currently using a distribution's latest version of Emacs.
The distribution is Ubuntu 12.04, precise pangolin, and the version of
Emacs it currently delivers is 23.3.1.  Is it 'simple' to add complex
text layout (CTL) for the Tai Tham script on non-Windows systems?

I hope that all I have to do is to:

1) Modify fontset.el to add the script.
2) Replace fontset.elc
in /usr/share/emacs/23.3/lisp/international/fontset.elc by the modified
version of fontset.el from the source description.
3) Augment the m17n library to use a HarfBuzz-compatible Tai Tham
font.  I have such a font, and it works fine with Emacs when CTL is not
needed.

One of my worries is that I may have to update characters.el and
recompile Emacs.

I already have a working Emacs input method for the Tai Tham script.

Richard.