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.
Brad King
Andrey Pokrovskiy