build: Only define OS_OBJECT_USE_OBJC with gcc

Recent versions of macOS use clang instead of gcc. The OS_OBJECT_USE_OBJC
define is only necessary when building with gcc. Let's not define it when
building with clang.

With this patch, I can successfully include GCD headers in QEMU when
building with clang.

Signed-off-by: Alexander Graf <graf@amazon.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-ID: <20230830161425.91946-2-graf@amazon.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

meson.build

@@ -223,7 +223,9 @@ qemu_ldflags = []
 if targetos == 'darwin'
   # Disable attempts to use ObjectiveC features in os/object.h since they
   # won't work when we're compiling with gcc as a C compiler.
-  qemu_common_flags += '-DOS_OBJECT_USE_OBJC=0'
+  if compiler.get_id() == 'gcc'
+    qemu_common_flags += '-DOS_OBJECT_USE_OBJC=0'
+  endif
 elif targetos == 'solaris'
   # needed for CMSG_ macros in sys/socket.h
   qemu_common_flags += '-D_XOPEN_SOURCE=600'