Macros can be defined from the compiler command line, with -DFOO, which would define the macro FOO. This is most often used for conditional compilation, e.g. a certain optimization that is known to work on some platforms but not on others. The build system can detect whether the optimization is viable an enable it using this kind of macro.
This is one of the few uses of macros that I believe can be used well. However, it's of course possible to abuse this feature as well.
