PrefixHeader.pch 是一个预编译的文件
New File >> Other >> PCH File 新建PrefixHeader.pch
Targets >> Bulid Setting >> Apple clang - Language
修改Precompile Prefix Header为YES；
Prefix Header路径设置为 $(SRCROOT)/项目名称/PrefixHeader.pch
代码：
#ifndef PrefixHeader_pch
#define PrefixHeader_pch
// Include any system framework and library headers here that should be included in all compilation units.
// You will also need to set the Prefix Header build setting of one or more of your targets to reference this file.
#ifdef DEBUG
#define MYLog(...) NSLog(__VA_ARGS__)
#else
#define MYLog(...)
#endif
#endif /* PrefixHeader_pch */
