Defined names can be used to to skip certain script lines dependent on previous define or -D command
line options. All script lines between ifdef and endif are
skipped if name was not defined before. Vice versa, all lines between IFNDEF and ENDIF are skipped if name was defined before. The instruction IFELSE reverses the line skipping or non-skipping.
C-Script only; for lite-C use #ifdef.
This way you can 'invent' new command line options for the engine, which have an arbitrary effect on the game.
define lores_d3d; // or start with -d lores_d3d
var video_mode = 8; // 1024x768
var video_mode = 6; // 640x480