Well, Visual Studio is obviously for Windows
The Code::Blocks one is the one that threw me, seems it was originally set up for Linux but hasn't been maintained and probably needs to be updated or removed.
Xcode, again, obviously for OS X.
Linux building is done via the autotools files. With a fresh checkout/export, run:
./autogen.sh [--enable-debug] [--enable-inferno]
make
Then you'll have fs2_open_[INF_][r, d] in your code folder. Copy it out and run 'make clean' to clear the slate, or reconfigure it with autogen.sh.
Not sure what you mean about the 'entire registry'. There are some files that are included in the Linux build process, and some that are only included in Windows, such as the audio files (taylor is cleaning that up though), the unix/win ini/registry stuff, etc. That's controlled at the project include level I think, but there are also many #ifdef _WIN32 checks throughout the code, etc. Just gotta look for it.