better way to handle required rebuild on library bump

[Eitan Adler]

The recent change to jpeg required a lot of changes to a lot of ports all
just to bump a version number.
It is easy to miss things this way and requires a lot of work and
downloading.

I propose that some kind of MAJORVERSION be stored in /var/db/ports. Then
when a library's MAJORVERSION is changed it will prompt a rebuild on any
port that relies on it will also get rebuilt.
Computer are *designed* handle these types of things