Since I got tired of being asked by people who are capable of doing it themselves but don't know how I figured it was better to just explain how it's done than to keep being asked.
I'm assuming you already know how to check out and compile FS2_Open and are just stuck on how to make an Inferno build. I'm also assuming you're using MSVC 2005 or 2008 (which is what most new Windows coders use).
On the left hand side of the program you'll see the solution explorer/class view/whatever. Right click on code, choose properties, select C++->Preprocessor. At the top you'll see Preprocessor Definitions and it will have a line that looks something like this
_WINDOWS;WIN32;_DEBUG;USE_OPENAL;_CRT_SECURE_NO_DEPRECATE;NO_DIRECT3D;_SECURE_SCL=0;_HAS_ITERATOR_DEBUGGING=0;
All you do is add, INF_BUILD; at the end of that line, after the last semi-colon (if there isn't one, add it). That particular project will now be an Inferno build. Unfortunately it's only that configuration of that project that this changes. If you look in the configuration option you'll probably have debug selected, swap to release and do the same thing to the other one.
The game will now build Inferno into the code, but it won't build FS2_Open builds until you repeat this process with the the other projects. So right click on Freespace2 and Fred2 and repeat the process (remembering to do it for both release and debug builds).
Now just rebuild everything (it is best to rebuild. I've seen MSVC completely **** up by failing to notice that a now has changes that need to be compiled in).
Once you're done you will have to reverse your steps and remove the INF_BUILD; entry if you want to build standard builds again.
