[RFC] adding a variable to .mk and Makefile.inc1 to point to top of the FreeBSD source tree

[Garrett Cooper]

Hi,
    A common pattern that I've seen at Isilon and something else that I've
wanted to have for a while is the ability to designate where the top of a
source tree was. This is important and helpful when dealing with source
files that build upon each other or depend on sources located in other
sections of the tree; contrib stuff needs to set .PATH appropriately to
point to sources at the top of the tree, sys stuff is riddled with S= in
order to point to where /sys, etc lives, we build upon FreeBSD within an
expected directory structure as well.
    I haven't come up with a name, but was wondering if this was a good
idea, and if so does anyone have any outstanding patches for this that can
be pushed into FreeBSD?
Thanks!
-Garrett