This enables locating the headers within the install-tree using the
dirs provided by `zeek-config --include_dir`.
To enable locating these headers within the build-tree, this change also
creates a 'build/src/include/zeek -> ..' symlink.
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.
