This makes it easier to install the headers, or to use them directly from
another software package with -I$(openflow)/include, without invading
the top-level include file namespace.
To enable SNAT, run configure with the "--enable-snat" flag. This has
only been tested with the 2.6.23 kernel...more diverse testing will follow.
Documentation and a cleaner build setup will also be in a future check-in.