FFHT: just expect benchmark to be installed (#5288)

Summary:
Pull Request resolved: #5288

The vendored benchmark setup didn't work right for me out of
the box, so I just changed the Makefile to use pkg-config to find
benchmark wherever it is installed.
ghstack-source-id: 242230775
exported-using-ghexport

Reviewed By: kimishpatel

Differential Revision: D60194969

fbshipit-source-id: 7c473b6dfdb845342126e667777e5a4cb8181e92

Author: swolchok