Creating support for multiple map engines is a good idea. I'll go through the code later, but I've already got some comments:

• Engine is kind of a bad name, because we already got a game engine. Maybe just MapGen or something like that?
• Isn't the directory structure too deep? Wouldn't cpp/mapgen/Mapgen.cpp and cpp/mapgen/static/static.cpp be good enough for now?
• Using usernames as filenames is a bad idea. Rename detrumi to heightmap.
• We shouldn't put too much work in the Terrain enum. There's ongoing work on the nyan specification, which will introduce nice data types for terrain and unit types.

Also note that my heightmap idea (based on the diamond-square algorithm) isn't working correctly yet, and may only work for the height generation step, and not the land generation (if we're following the same structure as the random map scripts in AoC, which we haven't decided yet).

Your code looks fine, just a few last things:

• mapgen.cpp and diamondsquare.cpp do the same thing (same with the header files). Did you intend to move it but left the old one there?
• There are some empty lines with just tabs in it, and some files don't have a newline at end of file. Look for some editor options to fix this sort of thing automatically (i.e. sublime text has ensure_newline_at_eof_on_save and trim_trailing_white_space_on_save)
• You have some very small fix commits, it would be nice if you did a rebase for those. Check out contributing.md for instructions on how to do a rebase. Marking the very small commits as fixup is good enough.

It looks good now, so I merged it.

(I accidentally merged it into detrumi:master instead of detrumi:random-maps, but I've fixed that afterwards)

@timohaas depends on your intentions, but your pull requests will be visible to a much broader audience if you submit them to the root project directly.

Of course i hope that detrumi will bring them upstream anyhow, but subforks are probably not that good, you'll also miss the CI builds then.