udatapath: use C++ linker

The nbee library is written in C++ so we have to use the C++ linker. Normally
automake detects which linker to use from the source file extensions, but
all of our sources are C. The trick suggested in the automake documentation [1]
is to add a fake C++ source file which doesn't actually need to exist.

Fixes #24.

[1]: http://www.gnu.org/software/automake/manual/html_node/Libtool-Convenience-Libraries.html
Merge pull request #25 from rlane/cxx-linker

udatapath: use C++ linker
Handle role requests with OFPCR_ROLE_NOCHANGE.
Print format for role messages
Add missing break to the case of role messages when packing.
Fix set_field when seting vlan id and pcp.
Fix the toal size of vlan packets.
Fix ethertype parsing of the last vlan tag
Fix matching QinQ ethernet type
Fix missing icmp parsing on customnetpdl.
Fix icmp parsing on dpctl.
Print remaining match error codes.
Fix switch crashing due to wrong checksum implementation.
Reworked the recalculation of ip checksum after a set_field action.
According to the spec, the action set can have more than one set_field action if they are of different oxm fields.
This commit fixes the wrong replacement of different set_field actions.
Use the right byte order function of in_port and metadata fields parsing
Update README
Update README.md
Update README.md
Update README.md
Update README.md
Change packet byte order to host and makes the match fields structure uniform.
Previously the parsed packet fields were stored and matched against the flow
using network byte order. It was causing problems because the conversion was
done by the packet match function, hindering the debug.
Another problem was that the ofl_match structure was using hash maps
with different match fields, preventing the use of an uniform function
to print and debug the packet match.
Fix ethertype parsing: Vlan types should not be parsed.
Correct pbb ethertype on customnetpdl.xml
Fix ip checksum: new ip value was being passed in the wrong byte order.
Some unused variables and printf debug cleaning.
Fix wrong reason for packet-in message generated by the table-miss flow-entry.
If the netdev_recv returns network down error in dp_ports_run, we set the port state OFPPS_LINK_DOWN.
Clean some unused debug.