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.
Yes, -L supports relative paths. The problem is that the go tool does not reinterpret the relative paths based on the directory they were defined in. This was the first approach I tried which would have eliminated the need to use pkg-config, but it doesn't work. :(

Arguably this is a bug in the go tool (or cgo).