Merge branch 'cmake-missing-bits'

This patchset adds next missing things (in compare to autotools):
- pkgconfig
- event_pthreads/event_openssl
- compile shared/static libraries always

And some fixes, because it will not build after fixing other things:
- export missing symbols for cmake (-fvisibility=hidden)

* cmake-missing-bits:
  cmake: support visibility for AppleClang too
  cmake: fix export absolute path and relative path and cleanup a bit
  cmake: generate and install pkgconfig files
  cmake: build SHARED and STATIC libraries (like autoconf does)
  cmake: add missing event_openssl/event_pthreads libraries
  Export symbols for -fvisibility=hidden (under cmake)

Refs: #246