use GIT_HOME to fetch tags
separate and reuse targets to clone dmd/druntime/phobos
use hyphen instead of dot for version suffix

.${LATEST} => -${LATEST}

- This matches the versioning scheme used by others
  (github, rpm spec, ...) and simplifies interop.
only perform checkout in git repos

- check for existence of .git subdir first
use /.cloned target for git cloning/checkout
Merge pull request #422 from MartinNowak/fix11579

[install] fix Issue 11579 - dlang.org repo can't be built without git