targets/dri-swrast: Convert to static/shared pipe-driver

Convert the final dri target to the single DRI (megadriver) library.
Cleanup all the automake leftovers from the conversion stage and
update the scons build.

v2: Link in llvmpipe, when applicable.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Tested-by: Rob Clark <robclark@freedesktop.org>
Tested-by: Thomas Helland <thomashelland90 at gmail.com>
Acked-by: Tom Stellard <thomas.stellard@amd.com>