This will make a number of things easier in the future, as well as (finally!)
avoiding the Id-smashing problem which has plagued developers for so long.
Boy, I'm glad we're not using sup anymore. This update would have been
insane otherwise.
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.
