This generator has been deprecated since CMake 3.3.  Remove it.
Update documentation, modules, and tests to drop content specific
to this generator.

Respect INTERFACE library property whitelist.  Check that a target has
type "UTILITY" before querying other properties.

Use file(GENERATE) to write the logging wrapper scripts to evaluate
generator expressions.  Use a per-config script names in case the
content varies by configuration.

Add an ExternalProjectSubdir test directory with a minimal test showing
use of ExternalProject_Add for a source tree in a subdirectory.  This
will allow us to easily add test external projects that cover specific
behavior where the client project must check results.