allow customizable PREFIX variable
allow customizable PREFIX variable

This change would let me specify my own PREFIX so that I can reuse
Makefile targets for building rpm packages.

Signed-off-by: Lokesh Mandvekar <46b23e80209bb43857d25fd87bafe9be89c7d33a@fedoraproject.org>

opencontainers/runc,dqminh/runc,xiaochenshen/runc,opencontainers/runc,kolyshkin/runc,rhatdan/runc,mikebrow/runc,opencontainers/runc,xiaochenshen/runc,dqminh/runc,estesp/runc,xiaochenshen/runc,opencontainers/runc,kolyshkin/runc,mikebrow/runc,dqminh/runc,rhatdan/runc,mikebrow/runc,estesp/runc,dqminh/runc,kolyshkin/runc,rhatdan/runc,kolyshkin/runc,estesp/runc,rhatdan/runc