Add a PREFIX variable so one can easily install wiggle into a different tree

Main use-case here: make install PREFIX=$HOME, to have it install into the user's
$HOME directory.
Use install(1) -d explicitly, avoiding the need for -D

-D isn't available on MacOS X by default, but there is not much point
here in using it: we can just create the directories first.
Merge branch 'allow-prefix-on-make' of https://github.com/ankon/wiggle