Describe FAQ MacOS X ProjectBuilder XCode and UNIX style build system differences here.
UNIX and Xcode build differences are well documented by Apple. See:
for step-by-step information on how to incorporate your existing UNIX build system with Xcode.
While you can continue to use Make (and friends) while working with your code in Xcode, you'll have to maintain the build system yourself. The biggest problem with an external build system is that you cannot access the key AppKit and Foundation frameworks automatically--- you'll need to add them by hand.
As per the instructions below, you'll need to add at least one External Build target along with Executable targets (that match the External Build Targets.) Once you've done these things, you can use the debugger. I highly recommend using the Xcode Help and Xcode Documentation. [Help > Xcode Help etc...]
In Xcode do the following: 1. Click File > New Project 2. Choose GNU Make 3. Click Project > Add Files... Adding a reference to the files in the project will not copy the files (unless you ask). This will keep your code under source control (so will copying, for that matter under subversion and cvs). But one tree is easier to deal with than two. 4. Add your new Build Targets (as per the instructions in the above document).