Problem with --data-dir command line option

I'm using source-highlight version 3.1.8 on OS X 10.11.3.
(I installed it using homebrew).

I am working on a couple of language definitions and I am running into a 
couple of problems. I'll detail one here and the other one in a separate 
email.


 From my reading of the documentation and the thread on this list from 
Aug 2014 
(http://lists.gnu.org/archive/html/help-source-highlight/2014-08/msg00000.html)....

My understanding is that if I add the --data-dir command line option, 
then I should just be able to add my new language files to the data-dir 
and an updated lang.map to the data-dir and it should work.

That is, I'm using the command line:

source-highlight --data-dir=/my/sourcehl -f esc256 -o STDOUT sample.4th

I have fourth.lang in /my/sourcehl and I put a copy of lang.map in this 
directory as well (adding the line 4th = forth.lang).

However, when I do this it complains that it can't find outlang.map.

So I copied that to my data directory. Then it complains that it can't 
find esc256.outlang.

I haven't had a chance to look through the source yet, but it seems that 
if you use --data-dir then it replaces the default directory, rather 
than supplements it.

Or am I doing something wrong?