DIY patching of OS X system binaries? So since nobody on Apple.SE, Unix.SE or other sites was able to help me with my little WWAN problem, I've taken a look at the configd source and I was wondering... suppose I'd want to give patching the code a shot, what dev tools would I need and is there something about OS X keeping me from replacing the vanilla configd binary with a patched one (on a new installation on an external drive so as not to render my OS X productive system unusable)?
You can use XCode - it's in the App Store, and free. https://itunes.apple.com/us/app/xcode/id497799835?mt=12

Download that whole directory, open up the XCode project: configd.xcodeproj

Then, you can build it using XCode.

You'll probably need to turn off the code-signing protections - Security and Privacy panel in System Prefs. Allow Apps downloaded from anywhere. Not sure if Apple has additional protection for system binaries.

Try it on a different drive, or in a VM and see what happens.