build: always use prefix=/ for tar-headers
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 <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>
Reviewed-By: Johan Bergström <3e38e5ce4283f06de4b83ad13af081abc5e16740@bergstroem.nu>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node