Here are instructions for unpacking the git archives of atlantis:

user@box:/tmp/atlantis$ ls
atlantis-2.1.2-1-git.tar.gz
user@box:/tmp/atlantis$ tar xf atlantis-2.1.2-1-git.tar.gz
user@box:/tmp/atlantis$ ls
atlantis-2.1.2-1-git.tar.gz  atlantis.bundle  mundus.bundle
user@box:/tmp/atlantis$ git init
Initialized empty Git repository in /tmp/atlantis/.git/
user@box:/tmp/atlantis$ git pull atlantis.bundle master
From atlantis.bundle
 * branch            master     -> FETCH_HEAD
user@box:/tmp/atlantis$ ls
atlantis-2.1.2-1-git.tar.gz  CMakeLists.txt  data           README  TODO
atlantis.bundle              COPYING         mundus.bundle  src
user@box:/tmp/atlantis$ cd src
user@box:/tmp/atlantis/src$ mkdir mundus
user@box:/tmp/atlantis/src$ cd mundus
user@box:/tmp/atlantis/src/mundus$ git init
Initialized empty Git repository in /tmp/atlantis/src/mundus/.git/
user@box:/tmp/atlantis/src/mundus$ git pull ../../mundus.bundle master
From ../../mundus.bundle
 * branch            master     -> FETCH_HEAD
user@box:/tmp/atlantis/src/mundus$ ls
CMakeLists.txt  doc  makewc.sh  spacedata  space.xml  src  TODO
user@box:/tmp/atlantis/src/mundus$ cd -
/tmp/atlantis
user@box:/tmp/atlantis$ cmake .
-- The C compiler identification is GNU
-- The CXX compiler identification is GNU
-- Check for working C compiler: /usr/bin/gcc
-- Check for working C compiler: /usr/bin/gcc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
# ... stripped annoying CMake warnings ...
-- Generating done
-- Build files have been written to: /tmp/atlantis
user@box:/tmp/atlantis$ make
Scanning dependencies of target mundus
[  4%] Building CXX object src/mundus/src/CMakeFiles/mundus.dir/creator.o
[  8%] Building CXX object src/mundus/src/CMakeFiles/mundus.dir/dump.o
[ 12%] Building CXX object src/mundus/src/CMakeFiles/mundus.dir/mundus.o
[ 16%] Building CXX object src/mundus/src/CMakeFiles/mundus.dir/object.o
[ 20%] Building CXX object src/mundus/src/CMakeFiles/mundus.dir/mstring.o
[ 25%] Building CXX object src/mundus/src/CMakeFiles/mundus.dir/error/error.o
[ 29%] Building CXX object src/mundus/src/CMakeFiles/mundus.dir/memory/refcount.o
[ 33%] Building CXX object src/mundus/src/CMakeFiles/mundus.dir/resource/libexpat.o
[ 37%] Building CXX object src/mundus/src/CMakeFiles/mundus.dir/resource/parser.o
[ 41%] Building CXX object src/mundus/src/CMakeFiles/mundus.dir/resource/resource.o
[ 45%] Building CXX object src/mundus/src/CMakeFiles/mundus.dir/graphics/event.o
[ 50%] Building CXX object src/mundus/src/CMakeFiles/mundus.dir/graphics/graphics.o
[ 54%] Building CXX object src/mundus/src/CMakeFiles/mundus.dir/graphics/object.o
[ 58%] Building CXX object src/mundus/src/CMakeFiles/mundus.dir/graphics/position.o
[ 62%] Building CXX object src/mundus/src/CMakeFiles/mundus.dir/graphics/libsdl/libsdl.o
Linking CXX static library libmundus.a
[ 62%] Built target mundus
Scanning dependencies of target atlantis
[ 66%] Building CXX object src/CMakeFiles/../atlantis.dir/artwork.o
[ 70%] Building CXX object src/CMakeFiles/../atlantis.dir/game.o
[ 75%] Building CXX object src/CMakeFiles/../atlantis.dir/hexagon.o
[ 79%] Building CXX object src/CMakeFiles/../atlantis.dir/main.o
[ 83%] Building CXX object src/CMakeFiles/../atlantis.dir/object.o
[ 87%] Building CXX object src/CMakeFiles/../atlantis.dir/pathfind.o
[ 91%] Building CXX object src/CMakeFiles/../atlantis.dir/select.o
Linking CXX executable ../atlantis
[ 91%] Built target ../atlantis
Scanning dependencies of target space
[ 95%] Building CXX object src/mundus/src/CMakeFiles/../space.dir/space/space.o
[100%] Building CXX object src/mundus/src/CMakeFiles/../space.dir/space/ship.o
Linking CXX executable ../space
[100%] Built target ../space
user@box:/tmp/atlantis$ ls
atlantis                     CMakeFiles           data           src
atlantis-2.1.2-1-git.tar.gz  cmake_install.cmake  Makefile       TODO
atlantis.bundle              CMakeLists.txt       mundus.bundle
CMakeCache.txt               COPYING              README
user@box:/tmp/atlantis$ ./atlantis
user@box:/tmp/atlantis$