I use git sub-trees. Actually I love the thing. They give you a 'linear' history, and allow you to merge/pull/push into their original tree, keeping the history (if you require it).
Never heard of them (well, probably in passing); will look into is. Thanks!
What's the alternative? Managing all dependencies by an external dependency manager does not exactly reduce complexity (if you're not within a closed ecosystem like Java + Maven that has a mature, de-facto standard dependency manager; npm might count, too).
It's absolutely not feasible for C++ projects; all projects that do this have horrible hacks upon hacks to fetch and mangle data and usually require gratuitous "make clean"s to untangle.