basic/macros: clang 3.5 doesn't support alloc_size

The attribute was removed in commit c047507 in the clang repository as it
was never properly implemented anyway. Avoid using the attribute with
clang because it generates a ton of annoying warnings.

src/basic/macro.h

@@ -26,7 +26,11 @@
 #include <sys/types.h>
 
 #define _printf_(a,b) __attribute__ ((format (printf, a, b)))
-#define _alloc_(...) __attribute__ ((alloc_size(__VA_ARGS__)))
+#ifdef __clang__
+#  define _alloc_(...)
+#else
+#  define _alloc_(...) __attribute__ ((alloc_size(__VA_ARGS__)))
+#endif
 #define _sentinel_ __attribute__ ((sentinel))
 #define _unused_ __attribute__ ((unused))
 #define _destructor_ __attribute__ ((destructor))