Enables the export(PACKAGE) command when CMP0090 is set to NEW.

The export(PACKAGE) command does nothing by default. In some cases it is desirable to write to the user package registry, so the CMAKE_EXPORT_PACKAGE_REGISTRY variable may be set to enable it.

If CMP0090 is not set to NEW this variable does nothing, and the CMAKE_EXPORT_NO_PACKAGE_REGISTRY variable controls the behavior instead.

See also Disabling the Package Registry.