They can't be used when evaluating link libraries, but they can be
used for include directories and compile definitions. Later they can
be used for compile options.
This generator expression joins a list with a separator. The separator
may contain arbitrary content, such as commas, which is ordinarily a
delimiter in the generator expression syntax.
Because the main file for the dummy-executable and the actual compile
test were both called main.cpp, they were overwriting each other during
in-source builds.
Stephen Kelly
Brad King