参考：http://stackoverflow.com/questions/24158648/why-isnt-projectname-prefix-pch-created-automatically-in-xcode-6

Without the question if it is proper or not, you can add PCH file manually:

• Add new PCH file to the project: New file > Other > PCH file.
• At the Target's Build Settings option, set the value of Prefix Header to your PCH file name, with the project name as prefix (i.e. for project named TestProject and PCH file named MyPrefixHeaderFile, add the value TestProject/MyPrefixHeaderFile.pch to the plist).
• TIP: You can use things like $(SRCROOT) or $(PROJECT_DIR) to get to the path of where you put the .pch in the project.
• At the Target's Build Settings option, set the value of Precompile Prefix Header to YES.

If you decide to add a .pch file manually and you want to use Objective-C just like before xCode 6 you will also have to import UIKit and Foundation frameworks in the .pch file. Otherwise you will have to import these frameworks manually in each header file. You can add the following code anyway as it tests for the language used:

#ifdef __OBJC__
    #import <UIKit/UIKit.h>
    #import <Foundation/Foundation.h>
#endif