+/* Only enable __attribute__() for compilers known to support it. */
+#if defined(__clang__)
+# define clang_attr(x) __attribute__(x)
+# define gcc_attr(x) /**/
+#elif __GNUC__
+# define clang_attr(x) /**/
+# define gcc_attr(x) __attribute__(x)
+#else
+# define clang_attr(x) /**/
+# define gcc_attr(x) /**/