driver by default but can also use the llvmpipe driver that use LLVM
for code generation if available.
Not yet linked to the build as it depends on pthreads and we don't know
yet how to handle the switch from the default 'swrast' driver, but having
it in tree will help testing and debugging the remaining issues.
Tested by ajacoutot@ and matthieu@, looks ok to matthieu@
