Xcode中的，都是用于 条件编译时，所需要定义变量时，有两个：
Active Compilation Conditions和Preprocessor Macros
两者有何区别
Active Compilation Conditions vs Preprocessor Macros
Super Preprocessor Directives with Xcode 8 – Derrick Ho – Medium
原来是：
对于OBJC，用：Preprocessor Macros
对于SWIFT，用：Active Compilation Conditions
xcode – #ifdef replacement in the Swift language – Stack Overflow
“
SWIFT_ACTIVE_COMPILATION_CONDITIONS
“Active Compilation Conditions” is a new build setting for passing conditional compilation flags to the Swift compiler.”
是Xcode8中新增的。
Using Swift with Cocoa and Objective-C (Swift 4): Interacting with C APIs
“Preprocessor Directives
The Swift compiler does not include a preprocessor. Instead, it takes advantage of compile-time attributes, conditional compilation blocks, and language features to accomplish the same functionality. For this reason, preprocessor directives are not imported in Swift.”
swift Active Compilation Conditions
Xcode 8: New build settings and analyzer improvements – miqu.me
之前swift中定义自定义变量时，都是放在OTHER_SWIFT_FLAGS中，现在都可以改用SWIFT_ACTIVE_COMPILATION_CONDITIONS的Active Compilation Conditions了。
xcode – Any way to do true conditional compilation in Swift 3? – Stack Overflow
“Active Compilation Conditions is a new build setting for passing conditional compilation flags to the Swift compiler. Each element of the value of this setting passes to swiftc prefixed with -D, in the same way that elements of Preprocessor Macros pass to clang with the same prefix. (22457329)”
【总结】
- 在之前OC时代（编译器是clang），条件编译所用到的变量定义，都是通过：GCC_PREPROCESSOR_DEFINITIONS的Preprocessor Macros去定义的；
- 后来swift中（编译器是swiftc），最开始时是用：OTHER_SWIFT_FLAGS的Other Swift Flags中定义的（加上-DXXX）
- Xcode 8中又更新为，换用SWIFT_ACTIVE_COMPILATION_CONDITIONS的Active Compilation Conditions去定义变量XXX即可（不需要加-D）。
此处，Xcode9中，当前代码是swift，用的编译系统也是swift，所以变量定义是用Active Compilation Conditions，而不是Preprocessor Macros。
