Improvements and cleanup to the Makefile

Change from rsyncing the _target directory to symlinking it instead.

This approach is used by Gitaly and minimises the risk of editing the rsync'ed file in the target directory, which can then get lost.

• Prep work for linting in !293 (merged)
• Symlinking instead of rsyncing
• Added missing .PHONY targets
• Switched to := variable assignment early evaluation to ensure build times are the same for all targets
• Migrated from { curly brackets } to more Makefile-like ( round brackets )