Makefile: improve experience when VERSION is unset
Makefile: improve experience when VERSION is unset

When you are building outside a git repo, you'd get a nasty message
that makes it looks like everything failed:

  fatal: Not a git repository (or any parent up to mount point /)
  Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).

Also, the "unknown version" alternative would not trigger because
VERSION is set to an empty string.

This patch fixes both issues.

rfjakob/earlyoom,rfjakob/earlyoom,rfjakob/earlyoom,rfjakob/earlyoom