-ObjC：
This flag causes the linker to load every object file in the library that defines an Objective-C class or category.
While this option will typically result in a larger executable (due to additional object code loaded into the application),
it will allow the successful creation of effective Objective-C static libraries that contain categories on existing classes.
这个flag告诉链接器把库中定义的Objective-C类和Category都加载进来。这样编译之后的app会变大（因为加载了其他的objc代码进来）。
但是如果静态库中有类和category的话只有加入这个flag才行。
-all_load
IMPORTANT: For 64-bit and iPhone OS applications, there is a linker bug that prevents -ObjC from loading objects files from static libraries that contain only categories and no classes.
The workaround is to use the -all_load or -force_load flags. -all_load forces the linker to load all object files from every archive it sees, even those without Objective-C code.
-force_load is available in Xcode 3.2 and later. It allows finer grain control of archive loading. Each -force_load option must be followed by a path to an archive,
and every object file in that archive will be loaded.
这个flag是专门处理-ObjC的一个bug的。用了-ObjC以后，如果类库中只有category没有类的时候这些category还是加载不进来。变通方法就是加入-all_load或者-force-load。-all_laod会强制
链接器把目标文件都加载进来，即使没有objc代码。-force_load在xcode3.2后可用。但是-force_load后面必须跟一个只想静态库的路径。
-force_load
When someone uses "-all_load", every imported library is forced into memory whether its needed or not. A much better technique is "-force_load":
-force_load $(BUILT_PRODUCTS_DIR)/libarc.a
This insures that ONLY your library is loaded, not every library.
http://developer.apple.com/mac/library/qa/qa2006/qa1490.html