Revert commit v3.0.0-rc1~175^2~20 (add_dependencies: Disallow use with
INTERFACE_LIBRARY, 2013-12-25). Teach our dependency analysis to
transitively follow INTERFACE target utility dependencies as was done or
IMPORTED targets in commit v2.8.6~127^2~1 (Allow add_dependencies() on
imported targets, 2010-11-19). Extend the InterfaceLibrary test with a
case to cover header generation for a header-only INTERFACE library via
a custom target.
Brad King