Clang doesn't define `__GNUC__` in MSVC compatibility mode, but it still provides the functionality typically associated with it.
-- v2: oleaut32: Remove __GNUC__ check. ntoskrnl: Remove __GNUC__ check. ntdll/tests: Remove __GNUC__ checks. ntdll: Remove __GNUC__ checks. krnl386: Remove __GNUC__ check.