which takes entries of the format <make-definition-name>=<pkgname>. This
has not been added to MAKEFLAGS because (a) premature optimisation is the
root of all evil, and (b) because the .for loop used to implement this
shows the wrong results when multiple prefices are evaluated.
Modify all the package Makefiles to use EVAL_PREFIX, thereby simplifying
them considerably.
ALso simplify the logic to calculate the prefix as well.
78748d628a