avoiding "Portfile changed since last build; discarding previous state."

[Jonathan Stickel]

When attempting to debug a portfile, every change to the portfile 
results in the message:

Portfile changed since last build; discarding previous state.

the build folder is deleted, and build process starts over.  Is there 
anyway to avoid this?  It is common to debug problems with the destroot 
phase, and I would like to avoid re-compiling all the time.

Thanks,
Jonathan