build: only define HAVE_SNMP

NetSNMP is the only SNMP implementation for Quagga. We don't need two
different symbols.
diff --git a/configure.ac b/configure.ac
index 80f65b9..c3c651f 100755
--- a/configure.ac
+++ b/configure.ac
@@ -1370,8 +1370,7 @@
         LIBS="${LIBS} -lcrypto";
     fi
     AC_CHECK_LIB(netsnmp, asn_parse_int,
-    	[AC_DEFINE(HAVE_NETSNMP,,Net SNMP) 
-    	 AC_DEFINE(HAVE_SNMP,,SNMP)
+    	[AC_DEFINE(HAVE_SNMP,,SNMP)
     	 LIBS="${LIBS} -lnetsnmp"],
     	[AC_MSG_ERROR([--enable-snmp given, but cannot find support for SNMP])])