From grefcount.h, in order to disable warnings when G_DISABLE_CHECKS is
defined, we need to make use of the GCC-isque-only __extension__ for it
to work.  This means that in the saturation tests, we can only expect no
warnings to show if we are building on GCC-isque compilers with
G_DISABLE_CHECKS defined.

This will fix the refcount-macro test on Visual Studio builds at least.