Avoid polluting CFLAGS for mod_snmp

Putting `net-snmp-config --cflags` into CFLAGS causes major pollution;
it overrides optimization and debugging levels, warnings, and more.
While normally we do want to automatically locate library headers,
there has to be a better way to do this.  libsnmp is normally in the
usual place and doesn't need special handling.  Perhaps people with
libsnmp in a weird place should just need to add the -I flag to their
CFLAGS before build.

src/mod/event_handlers/mod_snmp/Makefile

 include ../../../../build/modmake.rules
 
-LOCAL_CFLAGS=`net-snmp-config --cflags`
 LOCAL_LDFLAGS=-lnetsnmpmibs -lnetsnmpagent -lnetsnmp
 LOCAL_OBJS=subagent.o