build: always use prefix=/ for tar-headers

Defaults to /usr/local otherwise, which isn't helpful for node-gyp,
nor standard assumptions about how header directories should be
structured.

PR-URL: https://github.com/nodejs/io.js/pull/2082


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>