If you're a Github user you might wanna use a fork instead so you can have your own remote repository (you can't push to the official OpenXcom repository without permission) and easily send in pull requests.
OpenXcom is a cross-platform game so it's possible to compile it on a variety of systems and project files for all the popular IDEs are included. There's also a Doxygen documentation available.
Don't forget to copy the X-Com resources to your bin folder as shown in Installing.
OpenXcom requires the following libraries to compile:
- SDL (libsdl1.2)
- SDL_mixer (libsdl-mixer1.2)
- SDL_gfx (libsdl-gfx1.2), version 2.0.22 or later
- yaml-cpp, version 0.2.6 or later