Rely on evaluation in cmCustomCommandGenerator for the generators.
When tracing target dependencies, depend on the union of dependencies
for all configurations.
Generator expressions are supported in many places and are a distinct
concept worthy of their own manual page. The old builtin documentation
was previously represented by preprocessor macros to generate it into
each place that supports them. Factor out the duplicate content into a
dedicated cmake-generator-expressions manual page and reference it from
each original location.
Stephen Kelly
Brad King
Kitware Robot