Linking fails on PCC

[dargueta]

Despite having __GNUC__ set to 4, the standard library of the latest version of PCC doesn't have __sync_bool_compare_and_swap(). The simplest solution would be to explicitly exclude it:

#elif defined(__GNUC__) && (__GNUC__ >= 4) && !defined(__PCC__)
//                                         ^^^^^^^^^^^^^^^^^^^^

I have a feeling there are other compilers that define __GNUC__ that aren't fully compatible with it (e.g. Clang's function attributes differ). This may bite us later.