snmp: link with Net-SNMP later

Some systems contain static libraries in the output of `net-snmp-config
--agent-libs`. It is not possible to build a convenience library with a
static library. Therefore, we just link with the appropriate libs in the
last step. This should fix #57.
redhat: enable SNMP support on RHEL 5 and CentOS 5 by default

This is related with the fix in the previous commit (ae7f4c2).