GNUstep base almost builds with clang

[David Chisnall]

The remaining issue is that clang (and llvm-gcc, for that matter) does  
not support the __builtin_apply() family of intrinsics.  These are  
quite unreliable on GCC, and it seems that they are not used when  
FFCall or FFI is supported, but are still compiled in.

mframe.m (which is really horrible) seems to be compiled  
unconditionally, even though it does not seem to be used when ffcall  
or libffi is provided.

NSInvocation.m provides default implementations in terms of  
__builtin_apply() and friends that are not used when libffi or ffcall  
is provided.  Is it possible to move these into a separate concrete  
subclass of NSInvocation along the lines of GSFFIInvocation/ 
GSFFCallInvocation and only compile them when no better mechanism is  
provided?

objc-gnu2next.m uses them in a function that is deprecated on the  
legacy Apple runtime and not supported by the modern Apple runtime.   
Does anyone actually use next_objc_msg_sendv()?

David