Xcode -- what is $(SRCROOT)?

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

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

It's the path to the directory containing the Xcode project.