Summary:
* use "Marble" and "Astro" as cmake target names,
  so 3rd-party lib users would do:
  find_package(Marble REQUIRED)
  find_package(Astro REQUIRED)
  target_link_libraries(foo Marble Astro) # include dirs imported by target
* only install headers & cmake config files on unbundled platforms for now

Reviewers: rahn, sanjibanb, shentey, nienhueser, #marble

Reviewed By: nienhueser, #marble

Differential Revision: https://phabricator.kde.org/D2364

# Conflicts:
#	src/lib/marble/CMakeLists.txt