Pass -DDINDEXER_ENABLED_BACKENDS="list,of,backends" to cmake to tell
which backends you want to be built. Currently available backends are
postgresql and redis. Default is postgresql.
Allowed strings in the comma separated list above are the names of the
directories in src/backends.
This is helpful so when running dindexer in a dev environment you don't
need to list all the possible paths to every backend. Symlinks to all
backends will all be in the same place.
This also adds a public dependency to machinery in the header-only
backend project. This is because the backend does need functionality
from machinery, such as FileRecordData and TigerHash.
