EnablingLogging

A common first "gotcha" in simulation scripts, the following macro must be declared to enable logging:

  #define NS3_LOG_ENABLE

This should happen before core-module.h is included. Note that macros may be defined from the command line in gcc, e.g.:

g++ -Wall -g -DNS3_LOG_ENABLE -o foo src/foo.cc -lns3

This is useful to enable/disable logging without having to edit any code.