Can't export nor install CMake project depending on absl

Depending on absl's through its CMake buildsystem means that one cannot export a depending project's local buildsystem nor install it because abseil doesn't do either. This prevents further downstream projects from, for example, using `find_package(mylib)` by setting `mylib_DIR` to a build directory of mylib, or from installing mylib (and thus its dependencies) to some local path and using that.