fix how to compile with SDL2, added SDL2_CONFIG to var.mk

Both 2014/maffiodo1 and 2018/giles now use ${SDL2_CONFIG}
to compile with SDL2.

var.mk

@@ -171,6 +171,7 @@ SCP= scp
 SCREEN= screen
 SCRIPT= script
 SDIFF= sdiff
+SDL2_CONFIG= sdl2-config
 SED= sed
 SEQ= seq
 SFTP= sftp