I'm confused on modules

[Luca Dionisi]

Hello everybody. I'm not involved in xorg development, just a curious user
with some programming backgroud.
I'm finding some difficulties in building xorg and mesa from cvs sources
... guess what? I want to try Xgl!

One of these points is the modules dir.
Googling a bit, I found few hints suggesting to build Mesa with
-DDEFAULT_DRIVER_DIR=\\\"${XORG_PREFIX}/lib/X11/modules/dri\\\"
But during the installation of xorg with the install script
(util/modular/build.sh) I can see it is putting stuff in
${XORG_PREFIX}/lib/xorg/modules

So my question is:
How many modules dir must exist?

Another: Who is going to provide the modules? I mean, is the flag
-DDEFAULT_DRIVER_DIR in the Mesa build telling where to put files
or where to find files that have previously built?

Thanks a lot.

--Luca