It's often useful in a ports system to set some make variables when using a
particular package; for instance, in GARstow it's currently necessary to
add to CFLAGS when using ncurses.
The FreeBSD ports handle this by having USE_SOMEPACKAGE
variables for each of the packages that do this, but this doesn't scale very
well.
I'd rather have a flags.mk in each port's directory that got
automatically included by GARstow from each port that's a dependency of the
current port; I could thus put the CFLAGS tweak into
sys/ncurses/flags.mk and have it automatically included in every
port that depends on sys/ncurses.