biology/bedtools: Fix Makefile to respect PREFIX

[jwb]

Summary

biology/bedtools: Fix Makefile to respect PREFIX
Approved by jrm (mentor) or wen (mentor)
Differential to be added to commit message

Test Plan

portlint -AC: looks fine
Passed poudriere on {10.4,11.1}-{amd64,i386}

[jrm]

PORTREVISION bump? I think package will change if PREFIX is set to something other than /usr/local/, right?

[jwb]

I think that's a good idea and certainly won't hurt. I'll add it to the commit. Thanks...