Also now uses CMake's ENABLE_EXPORTS target property for the zeek
executable to ensure symbols are visible to plugins. Prior to CMake
3.4, the policy was to export symbols by default for certain platforms,
but later versions need either the explicit target property or policy.
Most of these changes are either cmake-related or plugin-related.
Added a new test "plugins/legacy.zeek" to test that legacy Bro plugins
still work.
Also added a symlink bro-path-dev.in because some legacy Bro packages
won't install without it.
By default, OS X 10.11 does not include openssl headers.
Since building a Bro plugin #includes Bro headers, which #include openssl
headers, we need to tell cmake to find these so that the compiler
can use them.
