Export cmake targets for CORSIKA8 as framework

We need to export CORSIKA8 target and its dependencies to allow "find_package(CORSIKA8)", and "target_link_libraries(myProg CORSIKA8::CORSIKA8)"

changed milestone to %Project protoype structure 2 (refactorring)

added Build System Development labels

created merge request !306 (merged) to address this issue

mentioned in merge request !306 (merged)

changed milestone to %ICRC-2021

closed with merge request !306 (merged)

mentioned in commit e845cfd7