AFAIK, this cannot be set as the default or by alt-clicking an existing drawing.
The only method I know to set it before placing it is:
1. Go to a layout,
2. Select the Drawing tool,
3. Click on the layout,
4. The 'Place Drawing' dialog appears, where you can select "an external source",
5. Then press the "Browse" button and the "Link To" dialog appears,
6. On the "Link To" dialog you can find the "Store Path as Relative to Project". This is where you can check it or uncheck it.
Unfortunately this won't work for subsequent drawings, and AFAIK external drawings will always come in as absolute, so you'll need to repeat this procedure every time you want a Relative drawing.
You may also find
this helpful.
Cheers,
Link.