cmExportBuildFileGenerator: handle genex-wrapped source paths
Previously a fileset with $<$<CONFIG:Debug>:some_file>
would show up
as-is (with escaping) in the build directory export. Instead, evaluate
all fileset entries as generator expressions and list them as they are
similar to the installation information.
Cc: @kyle.edwards @craig.scott
Backport: release
Edited by Ben Boeckel