Idea for Mk/Uses snippet

[Kimmo Paasiala]

There are quite a few ports that require system sources to be present
at /usr/src.

Would it make sense to create Mk/Uses snippet (for example named as
Mk/Uses/system-sources) that checks for let's say /usr/src/Makefile
and throws an error if the file is not present?

A port that requires the system sources to be present (for example
sysutils/lsof) could then just say:

USES= ... system-sources ..

What do you think?

-Kimmo