https://developer.apple.com/library/mac/documentation/DeveloperTools/Reference/XcodeBuildSettingRef/1-Build_Setting_Reference/build_setting_ref.html
http://blog.csdn.net/phunxm/article/details/17044337
：环境变量宏（Build Setting Macros）引用格式：${MACRO}，同Build Phases Run Script中的语法。
PROJECT = HelloWorld
PROJECT_DIR =~/Projects/Learn Objective-C/HelloWorld
PROJECT_FILE_PATH =${PROJECT_DIR}/HelloWorld.xcodeproj
PROJECT_NAME = HelloWorld
SOURCE_ROOT =${PROJECT_DIR}
SRCROOT =${PROJECT_DIR}
SDKROOT = /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk
SDK_DIR = /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk
SDK_NAME = macosx10.9
Xcode 配置常用变量（SRCROOT, PROJECT_DIR, PROJECT_NAME）
原文：http://www.cnblogs.com/willbin/p/5260814.html