Rely on file creation and dependencies instead of .PHONY targets.
Files will be rebuilt when changing version as the main `Makefile` will
have been updated. All steps are re-done on version change.
When deleting patches, the '.prepare' step should be redone thanks
to the included 'patch-dep.inc' file (TODO TEST ME).
Implementation in order:
* '.git': means the repository has been cloned.
* '.git-downloaded': Fetches the wanted version
* '.git-prepared': will clean checkout the version and apply patches