adding a language (lilypond)

I've read the docs.
Very informative but I miss the most basic thing: how can I add a language?

I tried the following (no success):

- in src/lang.map I added:
ly = lilypond.lang

- I added src/lilypond.lang and put just one line:
comment start "%"

Then I run again make and make install, but the changes I made are not 
copied to /usr/local. I mean: lang.map misses my edit and there's no 
lilypond.lang

What I'm missing?

Thanks in advance,
Federico