MB-9504: Check for __GNUC__ instead of HAVE_VISIBILITY

Change-Id: I74848480d33ce14b50232b765e592c37ef370b53
Reviewed-on: http://review.couchbase.org/30227
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
MB-9504 Fixed invalid C89 code

Change-Id: Ia4f3b54b5e7cc89aaa65a3c234ba8bf8055c0d07
MB-9504 Transition to CMake

Change-Id: Ia99b910a5ae9b90e559f395b736288937a81e01f