better way to handle required rebuild on library bump

[Buganini]

I thought about this during last jpeg update:
http://www.mail-archive.com/freebsd-ports@freebsd.org/msg22476.html
http://www.mail-archive.com/freebsd-ports@freebsd.org/msg22501.html

but this need non-trivial work on mk, and somebody may consider it too
complicated.


--Buganini