PROJECT_DIR. Identifies the directory containing the project ( .xcodeproj ) $(PROJECT_DIR)/build is used as the create the default value for: Intermediate Build Files Path OBJROOT.
It's the path to the directory containing the Xcode project.
That's basically your project root directory defined by Xcode.
http://developer.apple.com/library/mac/#documentation/DeveloperTools/Reference/XcodeBuildSettingRef/1-Build_Setting_Reference/build_setting_ref.html#//apple_ref/doc/uid/TP40003931-CH3-SW38
SRCROOT
$(SRCROOT)(aka $(SOURCE_ROOT)) is a path to your location where a.xcodeproj is.
It is simple to check, just put it in a field and Xcode gives you a tip

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