Add folly header-only interface target.
Add folly header-only interface target.

OSVR/OSVR-Core,OSVR/OSVR-Core,godbyk/OSVR-Core,godbyk/OSVR-Core,godbyk/OSVR-Core,godbyk/OSVR-Core,godbyk/OSVR-Core,godbyk/OSVR-Core,OSVR/OSVR-Core,OSVR/OSVR-Core,OSVR/OSVR-Core,OSVR/OSVR-Core