installation question (version problem)

Hi,
I'm new to Linux.
I'm installing xvid because of avifile. I have two questions?
I have xvidcore-0.9.1.
What is the installation procedure?
In build/generic/, I did:
    ./configure
    make
    make install
Everything seems to be OK.

Q1. But when I do my avifile installation is says:
checking for xvid_init in -lxvidcore ... Header file and library are out of 
sync. Header file supports version 2.1 API and shared library supports 
version 2.0 API.

Did I do something wrong while installing xvid??

Q2. When I did "./configure" I got the msg
"configure: WARNING: nasm not found - Compiling generic sources only"
Is it safe to ignore this warning msg?

Thanks!

- jon

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