[Johnny got his gun]

#include <amxmodx>

#if !defined (BLARGH)
      #define BLARGH 1
#endif

#if defined BLARGH

public plugin_init()
{
        register_plugin("Test Plugin","0.2","dopy + jghg in collaboration")
}

#endif

Note that even if #include <amxmod> works, you should probably use #include <amxmodx>, if you don't plan on getting your plugin working on old amxmod...