Build log: https://gist.github.com/1144059
Seems to be related to including the wrong version of /usr/include/net-snmp/system/darwin.h (darwin10.h has the value, darwin11.h and darwin.h do not)
I'm having the exact same issue.
You might try taking this upstream to http://www.net-snmp.org/ too.
Filed upstream: https://sourceforge.net/tracker/?func=detail&aid=3393879&group_id=12694&atid=112694
Same issue here.
Is there a patch we can use for a fix?
Updated to 5.7.1 in dcf85f5 - MacPorts seems to package this version with no particular Lion fixes.
5.7.1 still doesn't work for me https://gist.github.com/1878393
Same problem with 5.7.1
$ git rev-parse HEAD
Fixed in 658e7f95b885a8f7cd6d1d2b4252d1e869e13935.