Fix compilation of seg.c.
1. "#ifdef CHECKLEVEL > CheckLevelMINIMAL" was bogus: the #ifdef should have been #if, but even then it would not be right, because in the CHECKLEVEL_DYNAMIC configuration, CHECKLEVEL is a variable rather than a preprocessor constant. So use the condition defined(AVER_AND_CHECK_ALL) instead.
2. The final CHECKL(addr == seg->limit) only makes sense if the loop was executed.
3. The variables used by the loop need to be inside the #if to avoid warnings about unused variables.
4. Add reference to the job from a comment.
defined(AVER_AND_CHECK_ALL) is a better condition for invalidating the colour tables.
Remove some unused headers.