Hack fix for RPM packaging (Why this is needed to actually fix it, is beyond me)

This solves the directory conflicts that arise from the RPM installing
folders like /usr or /usr/local. Why it does this I will never know.

Regardless, this was pretty frustrating to discover and then spend
several hours trying to fix. Next time someone says "Just use
CPACK_RPM_RELOCATABLE" DON'T LISTEN TO THEM. THEY LIE!