Check clang-cl for HAVE_GCC_ATTRIBUTE_WARN_UNUSED, too

Change-Id: I820ea4b3efc51a0464470a8a53d022602d635c81

configure.ac

@@ -6051,19 +6051,6 @@ if test "$GCC" = "yes"; then
         ], [AC_MSG_RESULT([no])])
     AC_LANG_POP([C++])
 
-    AC_MSG_CHECKING([whether $CXX supports __attribute__((warn_unused))])
-    AC_LANG_PUSH([C++])
-    save_CXXFLAGS=$CXXFLAGS
-    CXXFLAGS="$CXXFLAGS -Werror -Wunknown-pragmas"
-    AC_COMPILE_IFELSE([AC_LANG_SOURCE([
-            struct __attribute__((warn_unused)) dummy {};
-        ])], [
-            AC_DEFINE([HAVE_GCC_ATTRIBUTE_WARN_UNUSED],[1])
-            AC_MSG_RESULT([yes])
-        ], [AC_MSG_RESULT([no])])
-    CXXFLAGS=$save_CXXFLAGS
-    AC_LANG_POP([C++])
-
     AC_MSG_CHECKING([whether STL uses __attribute__((warn_unused))])
     AC_LANG_PUSH([C++])
     save_CXXFLAGS=$CXXFLAGS
@@ -6080,6 +6067,19 @@ if test "$GCC" = "yes"; then
     AC_LANG_POP([C++])
 fi
 
+AC_MSG_CHECKING([whether $CXX supports __attribute__((warn_unused))])
+AC_LANG_PUSH([C++])
+save_CXXFLAGS=$CXXFLAGS
+CXXFLAGS="$CXXFLAGS -Werror -Wunknown-pragmas"
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([
+        struct __attribute__((warn_unused)) dummy {};
+    ])], [
+        AC_DEFINE([HAVE_GCC_ATTRIBUTE_WARN_UNUSED],[1])
+        AC_MSG_RESULT([yes])
+    ], [AC_MSG_RESULT([no])])
+CXXFLAGS=$save_CXXFLAGS
+AC_LANG_POP([C++])
+
 AC_SUBST(HAVE_GCC_AVX)
 AC_SUBST(HAVE_GCC_STACK_PROTECTOR_STRONG)
 AC_SUBST(HAVE_GCC_BUILTIN_ATOMIC)