cmake: use APPEND during exporting targets (for old cmake)

cmake: use APPEND during exporting targets (for old cmake)

On centos with cmake 2.8.12.2:
  CMake Error at cmake/AddEventLibrary.cmake:92 (export):
    export called with target "event_extra_shared" which requires target
    "event_core_shared" that is not in the export list.

    If the required target is not easy to reference in this call, consider
    using the APPEND option with multiple separate calls.

But on newer cmake I guess everything is ok.

Fixes: 7182c2f561570cd9ceb704623ebe9ae3608c7b43 ("cmake: build SHARED
and STATIC libraries (like autoconf does)")