MB-10905: Make couchbase_server.sh work on OSX too.

Set DYLD_LIBRARY_PATH on OSX in same manner as we set
LD_LIBRARY_PATH on other platforms.

Change-Id: I58e2bdf80d2fc70ef089bace02f353625b12699f
Reviewed-on: http://review.couchbase.org/36297
Tested-by: Sriram Melkote <siri@couchbase.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>