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.am

@@ -3,6 +3,6 @@ MODNAME=mod_snmp
 
 mod_LTLIBRARIES = mod_snmp.la
 mod_snmp_la_SOURCES  = mod_snmp.c subagent.c
-mod_snmp_la_CFLAGS   = $(AM_CFLAGS) `net-snmp-config --cflags`
+mod_snmp_la_CFLAGS   = $(AM_CFLAGS)
 mod_snmp_la_LIBADD   = $(switch_builddir)/libfreeswitch.la -lnetsnmpmibs -lnetsnmpagent -lnetsnmp
 mod_snmp_la_LDFLAGS  = -avoid-version -module -no-undefined -shared