Allow modules to customize cmake package configuration files

Allows modules to add customized sections to the cmake package configuration files and sets a variable indicating whether this module is installed or not.

Reassigned to @oliver.sander

mentioned in merge request dune-grid!58 (closed)

mentioned in merge request ug/ug!3 (closed)

mentioned in commit b7243931

Status changed to merged

Mentioned in commit b7243931