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.
