We were not building the metadata before installing, plus vrt command was wrong

Package managers usually work on an exported source directory (i.e. not on git
repo), however vrt relies on git describe to guess the version that it is
building. This change makes it possible to set the SEMVER variable from outside
of the makefile, so package managers can avoid the dependency on git

ocamlmerlin uses them for several things, among them navigating the source