While Michael Dean has already given a pretty comprehensive answer, I will add one further piece of advice, as I assume your old developer isn't going to be around to do a hand over when the new developer starts:
- Sit down with the existing developer and work through compiling the
code from scratch, documenting the procedure yourself.
You don't need to understand the code to do this, you just need to step through setting up the development environment, checking out the source code, compiling it and running the test suite.
Don't let the developer sit at the computer - do every step yourself and document every detail, especially the but you probably shouldn't need to do that again steps.
This can all be a mechanical dummy's guide (i.e. download X, install it, then install Y & Z before checking out the code using the V option on the W menu etc.), but it could save your new developer weeks of messing around, trying to work out how the old developers build environment was set up.
If sitting down with your developer isn't possible, Kim Burgess's suggestion of getting the developer to take a screencast of the build process is an excellent one.
Ideally, you should give your developer a machine that has never been used for development, install your screencasting software and get them to go through all of the steps to build the full software.
In fact, even if you do sit down with your developer, it might be worth running some screencasting software in the background, while he is showing you. that way you will have a permanent record of what you did and what happened.