exttextcat: suppress -Qunused-arguments warning for Apple clang

Change-Id: I857ea5f37452e2b1242fc35e569191adfc5f90be
Reviewed-on: https://gerrit.libreoffice.org/15719Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>

external/libexttextcat/ExternalProject_exttextcat.mk

@@ -18,7 +18,8 @@ $(call gb_ExternalProject_get_state_target,exttextcat,build):
 		./configure --disable-shared --with-pic \
 			$(if $(CROSS_COMPILING),--build=$(BUILD_PLATFORM) --host=$(HOST_PLATFORM)) \
 		CFLAGS="$(CFLAGS) $(gb_VISIBILITY_FLAGS) $(gb_DEBUG_CFLAGS) $(gb_COMPILEROPTFLAGS) \
-		$(if $(filter AIX,$(OS)),-D_LINUX_SOURCE_COMPAT)" \
+			$(if $(COM_GCC_IS_CLANG),-Qunused-arguments) \
+			$(if $(filter AIX,$(OS)),-D_LINUX_SOURCE_COMPAT)" \
 		&& $(MAKE) \
 	)