There are only skeletons left here; they merely serve as a backup to
include the real versions under ${PORTSDIR}/Mk while we update the ports
tree to include them directly.
a few variables that could be used in the port Makefile for ".if exists()"
tests. bsd.port.post.mk defines the rest.
Note: if you define USE_X_PREFIX or USE_IMAKE, put it before including
bsd.port.pre.mk. These are the only two variables used in the first part.
In reality, bsd.port.pre.mk and bsd.port.post.mk just include bsd.port.mk
with special variables to turn part of it off.
