https://sites.google.com/site/buildllvmpipe/

Building llvmpipe on Windows
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Building
Deopendencies 
Mesa - http://www.mesa3d.org/
LLVM – http://llvm.org
CMake – http://www.cmake.org/
Microsoft Visual Studio 2008+
SCons – http://www.scons.org/
Python – http://www.python.org/
libxml2-python – http://xmlsoft.org/sources/win32/python/
MinGW – http://www.mingw.org/
Flex package
Bison packages
We're going to build everything from source

Preaparing LLVM 
Download source http://llvm.org/releases/ or http://llvm.org/svn/llvm-project/llvm/trunk
Generating projects for Visual Studio with CMake
Download and install CMake http://www.cmake.org/cmake/resources/software.html
Open source folder in CMake
Configure, with native compiler
Set necessary building flags 
CMAKE_INSTALL_PREFIX — specify folder where binaries will be put
LLVM_USE_CRT_RELEASE — set MT, as LLVM will be statically linked with llvmpipe, which we will build with MT flag (release build)
Apply configuration (don't forget this one, spent week to figure it out)
Generate projects
Compiling LLVM in  Visual Studio
Find LLVM.sln, in projects folder and open it with Visual Studio
Choose INSTALL install
Set configuration Release
Start building
Define location LLVM in corresponding environment variable: 

LLVM=C:\LLVM
Prepare building evironment for Gallium3D llvmpipe
Download and install Python
Download and install libxml2-python
Download and install SCons
Download and install MinGW
After installation open MSys console by running C:\MinGW\msys\1.0\msys.bat
Install flex and bison packages, executing following command:

mingw-get install msys-flex msys-bison
Add MinGW MSys location to system's path 

PATH=...;C:\MinGW\msys\1.0\bin;
Download Mesa source ftp://ftp.freedesktop.org/pub/mesa/ or git://anongit.freedesktop.org/mesa/mesa
Building Gallium3D llvmpipe
open console, and go to Mesa source folder
Start building: 

c:\Python27\scons.bat build=release llvm=yes libgl-gdi
SCons installs in Python's folder but doesn't set it in system, thus we need to specify full path.
In the end, we get  mesa\build\windows-x86\gallium\targets\libgl-gdi\opengl32.dll
If everything went right you're done :)
Congratulations!
Based on short stories:

Getting Started with the LLVM System using Microsoft Visual Studio -- http://llvm.org/docs/GettingStartedVS.html
Mesa: Compiling/Installing -- http://www.mesa3d.org/install.html
Mesa: Gallium llvmpipe driver -- http://www.mesa3d.org/llvmpipe.html