Find ucd-snmp include files more intelligently

ext/snmp/config.m4

@@ -25,9 +25,10 @@ AC_ARG_WITH(snmp,
     if test "$withval" = "yes"; then
       SNMP_INCDIR=/usr/local/include
       SNMP_LIBDIR=/usr/local/lib
-      test -d /usr/local/include/ucd-snmp && SNMP_INCDIR=/usr/local/include/ucd-snmp
-	  test -d /usr/include/ucd-snmp && SNMP_INCDIR=/usr/include/ucd-snmp
-	  test -d /usr/include/snmp && SNMP_INCDIR=/usr/include/snmp
+      test -f /usr/local/include/ucd-snmp/snmp.h && SNMP_INCDIR=/usr/local/include/ucd-snmp
+	  test -f /usr/include/ucd-snmp/snmp.h && SNMP_INCDIR=/usr/include/ucd-snmp
+	  test -f /usr/include/snmp/snmp.h && SNMP_INCDIR=/usr/include/snmp
+	  test -f /usr/include/snmp/include/ucd-snmp/snmp.h && SNMP_INCDIR=/usr/include/snmp/include/ucd-snmp
 	  test -f /usr/lib/libsnmp.a && SNMP_LIBDIR=/usr/lib
 	  test -f /usr/lib/libsnmp.so && SNMP_LIBDIR=/usr/lib
     else