There is a simple build failure after JDK-8280982 with older GCCs:

```
* For target support_native_java.desktop_libawt_xawt_screencast_pipewire.o:
In file included from /home/buildbot/worker/build-jdkX-debian9/build/src/java.desktop/unix/native/libpipewire/include/spa/buffer/buffer.h:12:0,
                 from /home/buildbot/worker/build-jdkX-debian9/build/src/java.desktop/unix/native/libpipewire/include/pipewire/stream.h:171,
                 from /home/buildbot/worker/build-jdkX-debian9/build/src/java.desktop/unix/native/libawt_xawt/awt/screencast_pipewire.h:36,
                 from /home/buildbot/worker/build-jdkX-debian9/build/src/java.desktop/unix/native/libawt_xawt/awt/screencast_pipewire.c:33:
/home/buildbot/worker/build-jdkX-debian9/build/src/java.desktop/unix/native/libpipewire/include/spa/utils/defs.h:61:24: error: "__clang_major__" is not defined [-Werror=undef]
 #elif __GNUC__ >= 7 || __clang_major__ >= 10
                        ^~~~~~~~~~~~~~~
```

There is an obvious fix for this: we need to check for `defined(__GNUC__)` explicitly before touching `__clang_major__`.

(Yes, GCC 6 is old; but we would like to make sure it builds until we run into hard to resolve build issues. This allows modern JDKs to be built in legacy enterprise environments for e.g. portable builds.)