Two things to bear in mind: First, most of the straight-line borders in Africa are in the north and cut through very sparsely inhabited desert [1]. Second, and perhaps more importantly, prior to the early 20th century, borders weren't typically controlled in the way they are now, so drawing a straight line though an inhabited area wouldn't affect much beyond taxes and other administrative issues.

[1] http://www.catsg.org/cheetah/07_map-centre/7_1_entire-range/...