[CMake] Install target after building it

[Robert Dailey]

I want to setup a target's header files to be copied to a separate
directory after that target is built. Any dependent targets will
reference the INSTALLED header files, so they must be copied after
that target is built and prior to any other targets (that depend on
it) that get built.

Is there a way to do this? Right now I use INSTALL( FILES ) but this
isn't target-specific. Thanks.