Rework build settings to allow raw "go" tool usage.

Moved cgo settings into a pkg-config file (storage/engine/engine.pc)
which is created from storage/engine/engine.pc.in. Change the cgo
settings to find rocksdb in the same location for both normal builds and
docker builds.

Various cleanups to the Makefile now that we no longer need to set CGO_*
env variables.

Delete the Vagrantfile. It's likely currently broken.
I thought -L and -I supported relative paths?