2.4 is designed to run on Asterisk 1.4 so I'm not sure why you are installing 1.6. I suspect you will have problems with the AGI and extensions.conf code due to the changes which the Asterisk developers made between 1.4 and 1.6.
In theory you don't need DAHDI if you aren't running any telephony boards, in practice you do, at least in 1.4, because it provides Asterisk's timing source. Without it, things like conference rooms won't work, although this may not be true in 1.6.
Kind Regards
S