scons: Avoid the unsupported option -Wno-c99-designator in MacOS

Change-Id: I4d95c75915b17531bdd6d9161eb266bb91cd7bef
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33775
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/SConstruct b/SConstruct
index d229ab8..4e47bda 100755
--- a/SConstruct
+++ b/SConstruct
@@ -440,7 +440,8 @@
                          # interchangeably.
                          '-Wno-mismatched-tags',
                          ])
-    if compareVersions(clang_version, "10.0") >= 0:
+    if sys.platform != "darwin" and \
+       compareVersions(clang_version, "10.0") >= 0:
         main.Append(CCFLAGS=['-Wno-c99-designator'])
 
     if compareVersions(clang_version, "8.0") >= 0: