I tried building swr instead of llvmpipe on Windows (as opengl32.dll),
using Scons. It works for llvmpipe, but I can't find the appropriate Scons options not script files. It looks like building swr is currently only supported on Linux using automake?
Support for this has recently been pushed. I believe you now can do
scons swr=1 libgl-gdi
to get the relevant libraries. You still have to use GALLIUM_DRIVER=swr in the environment for it to pick swr over llvmpipe.