Compiling programmes: How to use #ifdef?
 

I currently have a basic idea how linux works and how to compile and install a programme. Sometimes, the compiler gives an error on a command like this:

#ifdef SCHED_NORMAL
set_user_nice(current, -10);
#else
current->nice = -10;
#endif

Clearly the guy who wrote this had this problem (in this case a different kind of semantics) in mind and added it through compiler directives (N.B. this is really where my c++ knowledge ends). It acts as a switch, if you will. How do I set SCHED_NORMAL to true so that it compiles correctly? I tried looking in the Makefile, but I couldn't find it.

you'd need to compile it with -DSCHED_NORMAL in the gcc command. normally there is a cpp_flags variable in the makefile, or something along those lines.