Change to C99 and add weak references

test/alloc.h

@@ -8,7 +8,7 @@
 #include <stdlib.h>
 #include <string.h>
 
-#if __has_attribute(unused)
+#if defined(__has_attribute) && __has_attribute(unused)
 #    define UNUSED __attribute__((unused))
 #else
 #    define UNUSED
@@ -67,8 +67,8 @@ static UNUSED char *counting_strdup(const char *str) {
 }
 
 static UNUSED const RefcountAllocator COUNTING_ALLOCATOR = {
-    .malloc = counting_malloc,
-    .free = counting_free,
+    .malloc.no_data = counting_malloc,
+    .free.no_data = counting_free,
 };
 
 #endif