Use GNUInstallDirs.cmake

Summary: I don't know if GNU install dirs (e.g. lib64 instead of lib) would be preferred, but here's a PR for changing the install behavior if it is. Thoughts?

Note: This was originally based on HHVM-3.2 branch as [PR #3272](https://github.com/facebook/hhvm/pull/3272)

Closes https://github.com/facebook/hhvm/pull/3323

Reviewed By: ptarjan, kdm

Differential Revision: D1469517

Pulled By: svcscm