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.
Eliminate callers of cmMakefile::GetIncludeDirectories.
All callers of GetIncludeDirectories should go through the local generator
object.
Only the local generator calls cmTarget::GetIncludeDirectories directly.