Fix: Allow to override the variable BUILD_WITH_NETSNMP (#1481)

Otherwise, when the variable was set to FALSE|NO|0|OFF the if statement always set the variable to TRUE.
Now, it defaults to TRUE only if it was not set at all

CMakeLists.txt

@@ -145,9 +145,9 @@ if (NOT OPENVAS_RUN_DIR)
   set (OPENVAS_RUN_DIR "/run/ospd")
 endif (NOT OPENVAS_RUN_DIR)
 
-if (NOT BUILD_WITH_NETSNMP)
+if (NOT DEFINED BUILD_WITH_NETSNMP)
   set (BUILD_WITH_NETSNMP TRUE)
-endif (NOT BUILD_WITH_NETSNMP)
+endif (NOT DEFINED BUILD_WITH_NETSNMP)
 
 set (OPENVAS_DATA_DIR    "${DATADIR}/openvas")
 set (OPENVAS_STATE_DIR   "${LOCALSTATEDIR}/lib/openvas")