KVO implementation needs to be portable between compilers

[rfm]

Currently there are two KVO implementations, one for gcc and the gnu runtime, one for clang and the gnustep runtime, and while this gives us a good deal of portability it also gives us two sets of code to maintain and differences between platform which prevent complete portability of code using the library. The first stop to avoiding this would be to remove the dependency of the new implementation on the use of clang-specific blocks.