Summary:
On linux we didn't account for the `--final-install-prefix` argument
which meant that the binaries would always be rewritten to be relative to
the destdir.

This commit fixes that.

Refs: https://github.com/facebook/watchman/issues/760

(this doesn't deal with the compiled in statedir being in the scratch path though)

Reviewed By: simpkins

Differential Revision: D20659749

fbshipit-source-id: 1e8e198a58361882249c33a67f54a7d97b849257