Add alias target

[BDoignies]

PR Description

Just a quick addition for consistency between CPM/local install and System Wide install.

Checklist

• Unit-test of your feature with Catch.
• Doxygen documentation of the code completed (classes, methods, types, members...)
• Documentation module page added or updated.
• New entry in the ChangeLog.md added.
• No warning raised in Debug mode.
• All continuous integration tests pass (Github Actions)

[BDoignies]

@dcoeurjo Closing this PR.

The actual problem is due to fetching order of libraries that indeed makes no sense in DGtal. I'll open another one that will provide better fix ; but will rewrite lot of cmake...