This page describes how to build mesa with llvm support. 

First build llvm from svn with clang support: 

    svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm
    cd llvm
    ./configure --enable-pic
    make
    make install
    cd tools
    svn co http://llvm.org/svn/llvm-project/cfe/trunk clang
    cd clang
    make
    make install

Next build mesa with llvm support: 

    git clone git+ssh://marcheu@git.freedesktop.org/git/mesa/mesa
    cd mesa
    git checkout --track -b gallium-0.2 origin/gallium-0.2
    make linux-llvm

Mesa-7.9 [still unreleased as of 17-09-2010, but already branched from master] has configure switch for autoconf-based build system:  

    --enable-gallium-llvm