Project Builder uses command line tools to compile and link a target's files. By editing macros in the target's build settings, you can specify flags to pass to these tools.
To specify additional flags for command-line tools:
Project Builder appends the flags you specify to the flags that Project Builder passes to the tools. Note that Project Builder already passes flags to these tools. For example, it passes a flag to the compiler to create debugging information for certain files, and it passes a flag to the linker to specify the locations of the frameworks you add to a target.