Cleanup makefile library dependencies of mod_snmp

Calling out to net-snmp-config --agent-libs causes transitive
dependencies to get pulled in, but we don't need those -- a sensible
dynamic linker pulls those in automatically.  Trying to track the
transitive dependencies manually would be a losing battle.

People were recently hitting this on Debian sid/jessie, where libpci
is in the transitive dependency list but isn't otherwise one of our
build dependencies.

src/mod/event_handlers/mod_snmp/Makefile

 include ../../../../build/modmake.rules
 
 LOCAL_CFLAGS=`net-snmp-config --cflags`
-LOCAL_LDFLAGS=`net-snmp-config --agent-libs`
+LOCAL_LDFLAGS=-lnetsnmpmibs -lnetsnmpagent -lnetsnmp
 LOCAL_OBJS=subagent.o
 
 local_depend: $(LOCAL_OBJS)